[PATCH] merge: allow to pretend a merge into detached HEAD is made into a branch

[PATCH] merge: allow to pretend a merge into detached HEAD is made into a branch

[Junio C Hamano]

When a series of patches for a topic-B depends on having topic-A,
the workflow to prepare the topic-B branch would look like this:

    $ git checkout -b topic-B main
    $ git merge --no-ff --no-edit topic-A
    $ git am <mbox-for-topic-B

When topic-A gets updated, recreating the first merge and rebasing
the rest of the topic-B, all on detached HEAD, is a useful
technique.  After updating topic-A with its new round of patches:

    $ git checkout topic-B
    $ prev=$(git rev-parse 'HEAD^{/^Merge branch .topic-A. into}')
    $ git checkout --detach $prev^1
    $ git merge --no-ff --no-edit topic-A
    $ git rebase --onto HEAD $prev @{-1}^0
    $ git checkout -B @{-1}

This will

 (0) check out the current topic-B.
 (1) find the previous merge of topic-A into topic-B.
 (2) detach the HEAD to the parent of the previous merge.
 (3) merge the updated topic-A to it.
 (4) reapply the patches to rebuild the rest of topic-B.
 (5) update topic-B with the result.

without contaminating the reflog of topic-B too much.  topic-B@{1}
is the "logically previous" state before topic-A got updated, for
example.  At (4), comparison (e.g. range-diff) between HEAD and
@{-1} is a meaningful way to sanity check the result, and the same
can be done at (5) by comparing topic-B and topic-B@{1}.

But there is one glitch.  The merge into the detached HEAD done in
the step (3) above gives us "Merge branch 'topic-A' into HEAD", and
does not say "into topic-B".

Teach the "--detached-head-name=<branch>" option to "git merge" and
its underlying "git fmt-merge-message", to pretend as if we were
merging into <branch> (instead of HEAD) when they prepare the merge
message.  The pretend name honors the usual "into <target>"
suppression mechanism, which can be seen in the tests added here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/git-fmt-merge-msg.txt |  6 ++++-
 Documentation/git-merge.txt         |  7 +++++-
 builtin/fmt-merge-msg.c             |  4 ++++
 builtin/merge.c                     |  4 ++++
 fmt-merge-msg.c                     |  2 ++
 fmt-merge-msg.h                     |  1 +
 t/t6200-fmt-merge-msg.sh            | 35 ++++++++++++++++++++++++++++-
 7 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/Documentation/git-fmt-merge-msg.txt b/Documentation/git-fmt-merge-msg.txt
index 6793d8fc05..235276752f 100644
--- a/Documentation/git-fmt-merge-msg.txt
+++ b/Documentation/git-fmt-merge-msg.txt
@@ -9,7 +9,7 @@ git-fmt-merge-msg - Produce a merge commit message
 SYNOPSIS
 --------
 [verse]
-'git fmt-merge-msg' [-m <message>] [--log[=<n>] | --no-log]
+'git fmt-merge-msg' [-m <message>] [--detached-head-name <branch>] [--log[=<n>] | --no-log]
 'git fmt-merge-msg' [-m <message>] [--log[=<n>] | --no-log] -F <file>
 
 DESCRIPTION
@@ -44,6 +44,10 @@ OPTIONS
 	Use <message> instead of the branch names for the first line
 	of the log message.  For use with `--log`.
 
+--detached-head-name <branch>::
+	When merging into a detached HEAD, prepare the merge
+	message as if merging to the branch `<branch>` instead.
+
 -F <file>::
 --file <file>::
 	Take the list of merged objects from <file> instead of
diff --git a/Documentation/git-merge.txt b/Documentation/git-merge.txt
index e8cecf5a51..c369c1776a 100644
--- a/Documentation/git-merge.txt
+++ b/Documentation/git-merge.txt
@@ -12,7 +12,8 @@ SYNOPSIS
 'git merge' [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
 	[--no-verify] [-s <strategy>] [-X <strategy-option>] [-S[<keyid>]]
 	[--[no-]allow-unrelated-histories]
-	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>] [<commit>...]
+	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>]
+	[--detached-head-name <branch>] [<commit>...]
 'git merge' (--continue | --abort | --quit)
 
 DESCRIPTION
@@ -76,6 +77,10 @@ The 'git fmt-merge-msg' command can be
 used to give a good default for automated 'git merge'
 invocations. The automated message can include the branch description.
 
+--detached-head-name <branch>::
+	When merging into a detached HEAD, prepare the default merge
+	message as if merging to the branch `<branch>` instead.
+
 -F <file>::
 --file=<file>::
 	Read the commit message to be used for the merge commit (in
diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index 48a8699de7..255c45b17a 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -12,6 +12,7 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 {
 	const char *inpath = NULL;
 	const char *message = NULL;
+	char *detached_head_name = NULL;
 	int shortlog_len = -1;
 	struct option options[] = {
 		{ OPTION_INTEGER, 0, "log", &shortlog_len, N_("n"),
@@ -23,6 +24,8 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 		  DEFAULT_MERGE_LOG_LEN },
 		OPT_STRING('m', "message", &message, N_("text"),
 			N_("use <text> as start of message")),
+		OPT_STRING(0, "detached", &detached_head_name, N_("name"),
+			   N_("use <name> instead of detached HEAD")),
 		OPT_FILENAME('F', "file", &inpath, N_("file to read from")),
 		OPT_END()
 	};
@@ -56,6 +59,7 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 	opts.add_title = !message;
 	opts.credit_people = 1;
 	opts.shortlog_len = shortlog_len;
+	opts.detached_head_name = detached_head_name;
 
 	ret = fmt_merge_msg(&input, &output, &opts);
 	if (ret)
diff --git a/builtin/merge.c b/builtin/merge.c
index 5f0476b0b7..d0b624bd65 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -87,6 +87,7 @@ static int signoff;
 static const char *sign_commit;
 static int autostash;
 static int no_verify;
+static char *detached_head_name;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  NO_TRIVIAL },
@@ -286,6 +287,8 @@ static struct option builtin_merge_options[] = {
 	{ OPTION_LOWLEVEL_CALLBACK, 'F', "file", &merge_msg, N_("path"),
 		N_("read message from file"), PARSE_OPT_NONEG,
 		NULL, 0, option_read_message },
+	OPT_STRING(0, "detached", &detached_head_name, N_("name"),
+		   N_("use <name> instead of detached HEAD")),
 	OPT__VERBOSITY(&verbosity),
 	OPT_BOOL(0, "abort", &abort_current_merge,
 		N_("abort the current in-progress merge")),
@@ -1121,6 +1124,7 @@ static void prepare_merge_message(struct strbuf *merge_names, struct strbuf *mer
 	opts.add_title = !have_message;
 	opts.shortlog_len = shortlog_len;
 	opts.credit_people = (0 < option_edit);
+	opts.detached_head_name = detached_head_name;
 
 	fmt_merge_msg(merge_names, merge_msg, &opts);
 	if (merge_msg->len)
diff --git a/fmt-merge-msg.c b/fmt-merge-msg.c
index 5216191488..72b96d45e6 100644
--- a/fmt-merge-msg.c
+++ b/fmt-merge-msg.c
@@ -656,6 +656,8 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 		die("No current branch");
 	if (starts_with(current_branch, "refs/heads/"))
 		current_branch += 11;
+	else if (!strcmp(current_branch, "HEAD") && opts->detached_head_name)
+		current_branch = opts->detached_head_name;
 
 	find_merge_parents(&merge_parents, in, &head_oid);
 
diff --git a/fmt-merge-msg.h b/fmt-merge-msg.h
index f2ab0e0085..b5f3a5ec54 100644
--- a/fmt-merge-msg.h
+++ b/fmt-merge-msg.h
@@ -9,6 +9,7 @@ struct fmt_merge_msg_opts {
 	unsigned add_title:1,
 		credit_people:1;
 	int shortlog_len;
+	const char *detached_head_name;
 };
 
 extern int merge_log_config;
diff --git a/t/t6200-fmt-merge-msg.sh b/t/t6200-fmt-merge-msg.sh
index 06c5fb5615..b6fa76189e 100755
--- a/t/t6200-fmt-merge-msg.sh
+++ b/t/t6200-fmt-merge-msg.sh
@@ -573,7 +573,28 @@ test_expect_success 'merge-msg with "merging" an annotated tag' '
 	test_cmp expected .git/MERGE_MSG
 '
 
+test_expect_success 'merge --detached=<name>' '
+	test_when_finished "git checkout main" &&
+	git checkout -B side main &&
+	git commit --allow-empty -m "One step ahead" &&
+
+	git checkout --detach main &&
+	git merge --no-ff side &&
+	git show -s --format="%s" >full.0 &&
+	head -n1 full.0 >actual &&
+	# expect that HEAD is shown as-is
+	grep -e "Merge branch .side. into HEAD$" actual &&
+
+	git reset --hard main &&
+	git merge --no-ff --detached=main side &&
+	git show -s --format="%s" >full.1 &&
+	head -n1 full.1 >actual &&
+	# expect that we pretend to be merging to main, that is suppressed
+	grep -e "Merge branch .side.$" actual
+'
+
 test_expect_success 'merge.suppressDest configuration' '
+	test_when_finished "git checkout main" &&
 	git checkout -B side main &&
 	git commit --allow-empty -m "One step ahead" &&
 	git checkout main &&
@@ -590,7 +611,19 @@ test_expect_success 'merge.suppressDest configuration' '
 	git -c merge.suppressDest="ma?*[rn]" fmt-merge-msg <.git/FETCH_HEAD >full.3 &&
 	head -n1 full.3 >actual &&
 	grep -e "Merge branch .side." actual &&
-	! grep -e " into main$" actual
+	! grep -e " into main$" actual &&
+
+	git checkout --detach HEAD &&
+	git -c merge.suppressDest="main" fmt-merge-msg <.git/FETCH_HEAD >full.4 &&
+	head -n1 full.4 >actual &&
+	grep -e "Merge branch .side. into HEAD$" actual &&
+
+	git -c merge.suppressDest="main" fmt-merge-msg \
+		--detached=main <.git/FETCH_HEAD >full.5 &&
+	head -n1 full.5 >actual &&
+	grep -e "Merge branch .side." actual &&
+	! grep -e " into main$" actual &&
+	! grep -e " into HEAD$" actual
 '
 
 test_done
-- 
2.34.1-472-g213ab46be7

Re: [PATCH] merge: allow to pretend a merge into detached HEAD is made into a branch

[Sergey Organov]

Junio C Hamano <gitster@pobox.com> writes:

[...]

> Teach the "--detached-head-name=<branch>" option to "git merge" and
> its underlying "git fmt-merge-message", to pretend as if we were
> merging into <branch> (instead of HEAD) when they prepare the merge
> message.  The pretend name honors the usual "into <target>"
> suppression mechanism, which can be seen in the tests added here.

I think renaming destination branch in the commit message is useful in
general, and should not be limited to merging to detached head. Please
consider to introduce something like "--into-name=<name>" instead, that
will do this renaming unconditionally.

Also, being an advocate for eventual abandoning of "detached head" from
Git terminology, I'd prefer to see more neutral name for the option
anyway.

Thanks,
-- Sergey Organov

Re: [PATCH] merge: allow to pretend a merge into detached HEAD is made into a branch

[Ævar Arnfjörð Bjarmason]

On Fri, Dec 17 2021, Junio C Hamano wrote:

> +		OPT_STRING(0, "detached", &detached_head_name, N_("name"),
> +			   N_("use <name> instead of detached HEAD")),
> [...]
> +	OPT_STRING(0, "detached", &detached_head_name, N_("name"),
> +		   N_("use <name> instead of detached HEAD")),

Maybe OPT_STRING_F and PARSE_OPT_NONEG, or does --no-detached make
sense?

Re: [PATCH] merge: allow to pretend a merge into detached HEAD is made into a branch

[Derrick Stolee]

On 12/18/2021 5:27 AM, Ævar Arnfjörð Bjarmason wrote:
> 
> On Fri, Dec 17 2021, Junio C Hamano wrote:
> 
>> +		OPT_STRING(0, "detached", &detached_head_name, N_("name"),
>> +			   N_("use <name> instead of detached HEAD")),
>> [...]
>> +	OPT_STRING(0, "detached", &detached_head_name, N_("name"),
>> +		   N_("use <name> instead of detached HEAD")),
> 
> Maybe OPT_STRING_F and PARSE_OPT_NONEG, or does --no-detached make
> sense?

While sometimes undocumented, these "--no-" options allows calling an
alias that might have the positive version included while disabling the
option.

	git alias merge-main merge <lots-of-custom-things> --detached-head-name=main
	git merge-main --no-detached-head-name

So, I think using the standard OPT_STRING here is fine.

Thanks,
-Stolee

Re: [PATCH] merge: allow to pretend a merge into detached HEAD is made into a branch

[Junio C Hamano]

Sergey Organov <sorganov@gmail.com> writes:

> ...
> consider to introduce something like "--into-name=<name>" instead, that
> will do this renaming unconditionally.

Excellent.

In the scenario I illustrated in the proposed log message, I used a
throw-away temporary working area to prepare the history for a real
branch, and detached HEAD is the perfect match for such a temporary
working area.  But it should not be a crime to use a real temporary
branch for such use case.

Thanks.

Re: [PATCH] merge: allow to pretend a merge into detached HEAD is made into a branch

[Junio C Hamano]

Derrick Stolee <stolee@gmail.com> writes:

> While sometimes undocumented, these "--no-" options allows calling an
> alias that might have the positive version included while disabling the
> option.
>
> 	git alias merge-main merge <lots-of-custom-things> --detached-head-name=main
> 	git merge-main --no-detached-head-name
>
> So, I think using the standard OPT_STRING here is fine.

Exactly.  Not just 'is fine', but is more correct.  Thanks.

[PATCH v2] merge: allow to pretend a merge is made into a different branch

[Junio C Hamano]

When a series of patches for a topic-B depends on having topic-A,
the workflow to prepare the topic-B branch would look like this:

    $ git checkout -b topic-B main
    $ git merge --no-ff --no-edit topic-A
    $ git am <mbox-for-topic-B

When topic-A gets updated, recreating the first merge and rebasing
the rest of the topic-B, all on detached HEAD, is a useful
technique.  After updating topic-A with its new round of patches:

    $ git checkout topic-B
    $ prev=$(git rev-parse 'HEAD^{/^Merge branch .topic-A. into}')
    $ git checkout --detach $prev^1
    $ git merge --no-ff --no-edit topic-A
    $ git rebase --onto HEAD $prev @{-1}^0
    $ git checkout -B @{-1}

This will

 (0) check out the current topic-B.
 (1) find the previous merge of topic-A into topic-B.
 (2) detach the HEAD to the parent of the previous merge.
 (3) merge the updated topic-A to it.
 (4) reapply the patches to rebuild the rest of topic-B.
 (5) update topic-B with the result.

without contaminating the reflog of topic-B too much.  topic-B@{1}
is the "logically previous" state before topic-A got updated, for
example.  At (4), comparison (e.g. range-diff) between HEAD and
@{-1} is a meaningful way to sanity check the result, and the same
can be done at (5) by comparing topic-B and topic-B@{1}.

But there is one glitch.  The merge into the detached HEAD done in
the step (3) above gives us "Merge branch 'topic-A' into HEAD", and
does not say "into topic-B".

Teach the "--into-name=<branch>" option to "git merge" and its
underlying "git fmt-merge-message", to pretend as if we were merging
into <branch>, no matter what branch we are actually merging into,
when they prepare the merge message.  The pretend name honors the
usual "into <target>" suppression mechanism, which can be seen in
the tests added here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 
Range-diff:
1:  0ada838311 ! 1:  273df9ed2a merge: allow to pretend a merge into detached HEAD is made into a branch
    @@ Metadata
     Author: Junio C Hamano <gitster@pobox.com>
     
      ## Commit message ##
    -    merge: allow to pretend a merge into detached HEAD is made into a branch
    +    merge: allow to pretend a merge is made into a different branch
     
         When a series of patches for a topic-B depends on having topic-A,
         the workflow to prepare the topic-B branch would look like this:
    @@ Commit message
         the step (3) above gives us "Merge branch 'topic-A' into HEAD", and
         does not say "into topic-B".
     
    -    Teach the "--detached-head-name=<branch>" option to "git merge" and
    -    its underlying "git fmt-merge-message", to pretend as if we were
    -    merging into <branch> (instead of HEAD) when they prepare the merge
    -    message.  The pretend name honors the usual "into <target>"
    -    suppression mechanism, which can be seen in the tests added here.
    +    Teach the "--into-name=<branch>" option to "git merge" and its
    +    underlying "git fmt-merge-message", to pretend as if we were merging
    +    into <branch>, no matter what branch we are actually merging into,
    +    when they prepare the merge message.  The pretend name honors the
    +    usual "into <target>" suppression mechanism, which can be seen in
    +    the tests added here.
     
         Signed-off-by: Junio C Hamano <gitster@pobox.com>
     
    @@ Documentation/git-fmt-merge-msg.txt: git-fmt-merge-msg - Produce a merge commit
      --------
      [verse]
     -'git fmt-merge-msg' [-m <message>] [--log[=<n>] | --no-log]
    -+'git fmt-merge-msg' [-m <message>] [--detached-head-name <branch>] [--log[=<n>] | --no-log]
    ++'git fmt-merge-msg' [-m <message>] [--into-name <branch>] [--log[=<n>] | --no-log]
      'git fmt-merge-msg' [-m <message>] [--log[=<n>] | --no-log] -F <file>
      
      DESCRIPTION
    @@ Documentation/git-fmt-merge-msg.txt: OPTIONS
      	Use <message> instead of the branch names for the first line
      	of the log message.  For use with `--log`.
      
    -+--detached-head-name <branch>::
    -+	When merging into a detached HEAD, prepare the merge
    -+	message as if merging to the branch `<branch>` instead.
    ++--into-name <branch>::
    ++	Prepare the merge message as if merging to the branch `<branch>`,
    ++	instead of the name of the real branch to which the merge is made.
     +
      -F <file>::
      --file <file>::
    @@ Documentation/git-merge.txt: SYNOPSIS
      	[--[no-]allow-unrelated-histories]
     -	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>] [<commit>...]
     +	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>]
    -+	[--detached-head-name <branch>] [<commit>...]
    ++	[--into-name <branch>] [<commit>...]
      'git merge' (--continue | --abort | --quit)
      
      DESCRIPTION
    @@ Documentation/git-merge.txt: The 'git fmt-merge-msg' command can be
      used to give a good default for automated 'git merge'
      invocations. The automated message can include the branch description.
      
    -+--detached-head-name <branch>::
    -+	When merging into a detached HEAD, prepare the default merge
    -+	message as if merging to the branch `<branch>` instead.
    ++--into-name <branch>::
    ++	Prepare the default merge message as if merging to the branch
    ++	`<branch>`, instead of the name of the real branch to which
    ++	the merge is made.
     +
      -F <file>::
      --file=<file>::
    @@ builtin/fmt-merge-msg.c: int cmd_fmt_merge_msg(int argc, const char **argv, cons
      {
      	const char *inpath = NULL;
      	const char *message = NULL;
    -+	char *detached_head_name = NULL;
    ++	char *into_name = NULL;
      	int shortlog_len = -1;
      	struct option options[] = {
      		{ OPTION_INTEGER, 0, "log", &shortlog_len, N_("n"),
    @@ builtin/fmt-merge-msg.c: int cmd_fmt_merge_msg(int argc, const char **argv, cons
      		  DEFAULT_MERGE_LOG_LEN },
      		OPT_STRING('m', "message", &message, N_("text"),
      			N_("use <text> as start of message")),
    -+		OPT_STRING(0, "detached", &detached_head_name, N_("name"),
    -+			   N_("use <name> instead of detached HEAD")),
    ++		OPT_STRING(0, "into-name", &into_name, N_("name"),
    ++			   N_("use <name> instead of the real target branch")),
      		OPT_FILENAME('F', "file", &inpath, N_("file to read from")),
      		OPT_END()
      	};
    @@ builtin/fmt-merge-msg.c: int cmd_fmt_merge_msg(int argc, const char **argv, cons
      	opts.add_title = !message;
      	opts.credit_people = 1;
      	opts.shortlog_len = shortlog_len;
    -+	opts.detached_head_name = detached_head_name;
    ++	opts.into_name = into_name;
      
      	ret = fmt_merge_msg(&input, &output, &opts);
      	if (ret)
    @@ builtin/merge.c: static int signoff;
      static const char *sign_commit;
      static int autostash;
      static int no_verify;
    -+static char *detached_head_name;
    ++static char *into_name;
      
      static struct strategy all_strategy[] = {
      	{ "recursive",  NO_TRIVIAL },
    @@ builtin/merge.c: static struct option builtin_merge_options[] = {
      	{ OPTION_LOWLEVEL_CALLBACK, 'F', "file", &merge_msg, N_("path"),
      		N_("read message from file"), PARSE_OPT_NONEG,
      		NULL, 0, option_read_message },
    -+	OPT_STRING(0, "detached", &detached_head_name, N_("name"),
    -+		   N_("use <name> instead of detached HEAD")),
    ++	OPT_STRING(0, "into-name", &into_name, N_("name"),
    ++		   N_("use <name> instead of the real target")),
      	OPT__VERBOSITY(&verbosity),
      	OPT_BOOL(0, "abort", &abort_current_merge,
      		N_("abort the current in-progress merge")),
    @@ builtin/merge.c: static void prepare_merge_message(struct strbuf *merge_names, s
      	opts.add_title = !have_message;
      	opts.shortlog_len = shortlog_len;
      	opts.credit_people = (0 < option_edit);
    -+	opts.detached_head_name = detached_head_name;
    ++	opts.into_name = into_name;
      
      	fmt_merge_msg(merge_names, merge_msg, &opts);
      	if (merge_msg->len)
     
      ## fmt-merge-msg.c ##
     @@ fmt-merge-msg.c: int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
    + 
    + 	memset(&merge_parents, 0, sizeof(merge_parents));
    + 
    +-	/* get current branch */
    ++	/* learn the commit that we merge into and the current branch name */
    + 	current_branch = current_branch_to_free =
    + 		resolve_refdup("HEAD", RESOLVE_REF_READING, &head_oid, NULL);
    + 	if (!current_branch)
      		die("No current branch");
    - 	if (starts_with(current_branch, "refs/heads/"))
    +-	if (starts_with(current_branch, "refs/heads/"))
    ++
    ++	if (opts->into_name)
    ++		current_branch = opts->into_name;
    ++	else if (starts_with(current_branch, "refs/heads/"))
      		current_branch += 11;
    -+	else if (!strcmp(current_branch, "HEAD") && opts->detached_head_name)
    -+		current_branch = opts->detached_head_name;
      
      	find_merge_parents(&merge_parents, in, &head_oid);
    - 
     
      ## fmt-merge-msg.h ##
     @@ fmt-merge-msg.h: struct fmt_merge_msg_opts {
      	unsigned add_title:1,
      		credit_people:1;
      	int shortlog_len;
    -+	const char *detached_head_name;
    ++	const char *into_name;
      };
      
      extern int merge_log_config;
    @@ t/t6200-fmt-merge-msg.sh: test_expect_success 'merge-msg with "merging" an annot
      	test_cmp expected .git/MERGE_MSG
      '
      
    -+test_expect_success 'merge --detached=<name>' '
    ++test_expect_success 'merge --into-name=<name>' '
     +	test_when_finished "git checkout main" &&
     +	git checkout -B side main &&
     +	git commit --allow-empty -m "One step ahead" &&
    @@ t/t6200-fmt-merge-msg.sh: test_expect_success 'merge-msg with "merging" an annot
     +	grep -e "Merge branch .side. into HEAD$" actual &&
     +
     +	git reset --hard main &&
    -+	git merge --no-ff --detached=main side &&
    ++	git merge --no-ff --into-name=main side &&
     +	git show -s --format="%s" >full.1 &&
     +	head -n1 full.1 >actual &&
     +	# expect that we pretend to be merging to main, that is suppressed
    ++	grep -e "Merge branch .side.$" actual &&
    ++
    ++	git checkout -b throwaway main &&
    ++	git merge --no-ff --into-name=main side &&
    ++	git show -s --format="%s" >full.2 &&
    ++	head -n1 full.2 >actual &&
    ++	# expect that we pretend to be merging to main, that is suppressed
     +	grep -e "Merge branch .side.$" actual
     +'
     +
    @@ t/t6200-fmt-merge-msg.sh: test_expect_success 'merge.suppressDest configuration'
     +	grep -e "Merge branch .side. into HEAD$" actual &&
     +
     +	git -c merge.suppressDest="main" fmt-merge-msg \
    -+		--detached=main <.git/FETCH_HEAD >full.5 &&
    ++		--into-name=main <.git/FETCH_HEAD >full.5 &&
     +	head -n1 full.5 >actual &&
     +	grep -e "Merge branch .side." actual &&
     +	! grep -e " into main$" actual &&

 Documentation/git-fmt-merge-msg.txt |  6 ++++-
 Documentation/git-merge.txt         |  8 +++++-
 builtin/fmt-merge-msg.c             |  4 +++
 builtin/merge.c                     |  4 +++
 fmt-merge-msg.c                     |  7 +++--
 fmt-merge-msg.h                     |  1 +
 t/t6200-fmt-merge-msg.sh            | 42 ++++++++++++++++++++++++++++-
 7 files changed, 67 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-fmt-merge-msg.txt b/Documentation/git-fmt-merge-msg.txt
index 6793d8fc05..6f28812f38 100644
--- a/Documentation/git-fmt-merge-msg.txt
+++ b/Documentation/git-fmt-merge-msg.txt
@@ -9,7 +9,7 @@ git-fmt-merge-msg - Produce a merge commit message
 SYNOPSIS
 --------
 [verse]
-'git fmt-merge-msg' [-m <message>] [--log[=<n>] | --no-log]
+'git fmt-merge-msg' [-m <message>] [--into-name <branch>] [--log[=<n>] | --no-log]
 'git fmt-merge-msg' [-m <message>] [--log[=<n>] | --no-log] -F <file>
 
 DESCRIPTION
@@ -44,6 +44,10 @@ OPTIONS
 	Use <message> instead of the branch names for the first line
 	of the log message.  For use with `--log`.
 
+--into-name <branch>::
+	Prepare the merge message as if merging to the branch `<branch>`,
+	instead of the name of the real branch to which the merge is made.
+
 -F <file>::
 --file <file>::
 	Take the list of merged objects from <file> instead of
diff --git a/Documentation/git-merge.txt b/Documentation/git-merge.txt
index e4f3352eb5..ed0990621f 100644
--- a/Documentation/git-merge.txt
+++ b/Documentation/git-merge.txt
@@ -12,7 +12,8 @@ SYNOPSIS
 'git merge' [-n] [--stat] [--no-commit] [--squash] [--[no-]edit]
 	[--no-verify] [-s <strategy>] [-X <strategy-option>] [-S[<keyid>]]
 	[--[no-]allow-unrelated-histories]
-	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>] [<commit>...]
+	[--[no-]rerere-autoupdate] [-m <msg>] [-F <file>]
+	[--into-name <branch>] [<commit>...]
 'git merge' (--continue | --abort | --quit)
 
 DESCRIPTION
@@ -76,6 +77,11 @@ The 'git fmt-merge-msg' command can be
 used to give a good default for automated 'git merge'
 invocations. The automated message can include the branch description.
 
+--into-name <branch>::
+	Prepare the default merge message as if merging to the branch
+	`<branch>`, instead of the name of the real branch to which
+	the merge is made.
+
 -F <file>::
 --file=<file>::
 	Read the commit message to be used for the merge commit (in
diff --git a/builtin/fmt-merge-msg.c b/builtin/fmt-merge-msg.c
index 48a8699de7..8d8fd393f8 100644
--- a/builtin/fmt-merge-msg.c
+++ b/builtin/fmt-merge-msg.c
@@ -12,6 +12,7 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 {
 	const char *inpath = NULL;
 	const char *message = NULL;
+	char *into_name = NULL;
 	int shortlog_len = -1;
 	struct option options[] = {
 		{ OPTION_INTEGER, 0, "log", &shortlog_len, N_("n"),
@@ -23,6 +24,8 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 		  DEFAULT_MERGE_LOG_LEN },
 		OPT_STRING('m', "message", &message, N_("text"),
 			N_("use <text> as start of message")),
+		OPT_STRING(0, "into-name", &into_name, N_("name"),
+			   N_("use <name> instead of the real target branch")),
 		OPT_FILENAME('F', "file", &inpath, N_("file to read from")),
 		OPT_END()
 	};
@@ -56,6 +59,7 @@ int cmd_fmt_merge_msg(int argc, const char **argv, const char *prefix)
 	opts.add_title = !message;
 	opts.credit_people = 1;
 	opts.shortlog_len = shortlog_len;
+	opts.into_name = into_name;
 
 	ret = fmt_merge_msg(&input, &output, &opts);
 	if (ret)
diff --git a/builtin/merge.c b/builtin/merge.c
index ea3112e0c0..1ba5951d49 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -87,6 +87,7 @@ static int signoff;
 static const char *sign_commit;
 static int autostash;
 static int no_verify;
+static char *into_name;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  NO_TRIVIAL },
@@ -286,6 +287,8 @@ static struct option builtin_merge_options[] = {
 	{ OPTION_LOWLEVEL_CALLBACK, 'F', "file", &merge_msg, N_("path"),
 		N_("read message from file"), PARSE_OPT_NONEG,
 		NULL, 0, option_read_message },
+	OPT_STRING(0, "into-name", &into_name, N_("name"),
+		   N_("use <name> instead of the real target")),
 	OPT__VERBOSITY(&verbosity),
 	OPT_BOOL(0, "abort", &abort_current_merge,
 		N_("abort the current in-progress merge")),
@@ -1122,6 +1125,7 @@ static void prepare_merge_message(struct strbuf *merge_names, struct strbuf *mer
 	opts.add_title = !have_message;
 	opts.shortlog_len = shortlog_len;
 	opts.credit_people = (0 < option_edit);
+	opts.into_name = into_name;
 
 	fmt_merge_msg(merge_names, merge_msg, &opts);
 	if (merge_msg->len)
diff --git a/fmt-merge-msg.c b/fmt-merge-msg.c
index 5216191488..d25594545c 100644
--- a/fmt-merge-msg.c
+++ b/fmt-merge-msg.c
@@ -649,12 +649,15 @@ int fmt_merge_msg(struct strbuf *in, struct strbuf *out,
 
 	memset(&merge_parents, 0, sizeof(merge_parents));
 
-	/* get current branch */
+	/* learn the commit that we merge into and the current branch name */
 	current_branch = current_branch_to_free =
 		resolve_refdup("HEAD", RESOLVE_REF_READING, &head_oid, NULL);
 	if (!current_branch)
 		die("No current branch");
-	if (starts_with(current_branch, "refs/heads/"))
+
+	if (opts->into_name)
+		current_branch = opts->into_name;
+	else if (starts_with(current_branch, "refs/heads/"))
 		current_branch += 11;
 
 	find_merge_parents(&merge_parents, in, &head_oid);
diff --git a/fmt-merge-msg.h b/fmt-merge-msg.h
index f2ab0e0085..99054042dc 100644
--- a/fmt-merge-msg.h
+++ b/fmt-merge-msg.h
@@ -9,6 +9,7 @@ struct fmt_merge_msg_opts {
 	unsigned add_title:1,
 		credit_people:1;
 	int shortlog_len;
+	const char *into_name;
 };
 
 extern int merge_log_config;
diff --git a/t/t6200-fmt-merge-msg.sh b/t/t6200-fmt-merge-msg.sh
index 06c5fb5615..d861d7ca28 100755
--- a/t/t6200-fmt-merge-msg.sh
+++ b/t/t6200-fmt-merge-msg.sh
@@ -573,7 +573,35 @@ test_expect_success 'merge-msg with "merging" an annotated tag' '
 	test_cmp expected .git/MERGE_MSG
 '
 
+test_expect_success 'merge --into-name=<name>' '
+	test_when_finished "git checkout main" &&
+	git checkout -B side main &&
+	git commit --allow-empty -m "One step ahead" &&
+
+	git checkout --detach main &&
+	git merge --no-ff side &&
+	git show -s --format="%s" >full.0 &&
+	head -n1 full.0 >actual &&
+	# expect that HEAD is shown as-is
+	grep -e "Merge branch .side. into HEAD$" actual &&
+
+	git reset --hard main &&
+	git merge --no-ff --into-name=main side &&
+	git show -s --format="%s" >full.1 &&
+	head -n1 full.1 >actual &&
+	# expect that we pretend to be merging to main, that is suppressed
+	grep -e "Merge branch .side.$" actual &&
+
+	git checkout -b throwaway main &&
+	git merge --no-ff --into-name=main side &&
+	git show -s --format="%s" >full.2 &&
+	head -n1 full.2 >actual &&
+	# expect that we pretend to be merging to main, that is suppressed
+	grep -e "Merge branch .side.$" actual
+'
+
 test_expect_success 'merge.suppressDest configuration' '
+	test_when_finished "git checkout main" &&
 	git checkout -B side main &&
 	git commit --allow-empty -m "One step ahead" &&
 	git checkout main &&
@@ -590,7 +618,19 @@ test_expect_success 'merge.suppressDest configuration' '
 	git -c merge.suppressDest="ma?*[rn]" fmt-merge-msg <.git/FETCH_HEAD >full.3 &&
 	head -n1 full.3 >actual &&
 	grep -e "Merge branch .side." actual &&
-	! grep -e " into main$" actual
+	! grep -e " into main$" actual &&
+
+	git checkout --detach HEAD &&
+	git -c merge.suppressDest="main" fmt-merge-msg <.git/FETCH_HEAD >full.4 &&
+	head -n1 full.4 >actual &&
+	grep -e "Merge branch .side. into HEAD$" actual &&
+
+	git -c merge.suppressDest="main" fmt-merge-msg \
+		--into-name=main <.git/FETCH_HEAD >full.5 &&
+	head -n1 full.5 >actual &&
+	grep -e "Merge branch .side." actual &&
+	! grep -e " into main$" actual &&
+	! grep -e " into HEAD$" actual
 '
 
 test_done
-- 
2.34.1-472-g213ab46be7

Re: [PATCH v2] merge: allow to pretend a merge is made into a different branch

[Derrick Stolee]

On 12/20/2021 5:53 PM, Junio C Hamano wrote:

> Teach the "--into-name=<branch>" option to "git merge" and its
> underlying "git fmt-merge-message", to pretend as if we were merging
> into <branch>, no matter what branch we are actually merging into,
> when they prepare the merge message.  The pretend name honors the
> usual "into <target>" suppression mechanism, which can be seen in
> the tests added here.

I like the change to "--into-name".
 
> -	if (starts_with(current_branch, "refs/heads/"))
> +
> +	if (opts->into_name)
> +		current_branch = opts->into_name;
> +	else if (starts_with(current_branch, "refs/heads/"))
>  		current_branch += 11;

I briefly considered whether this 'else if' should just be an
'if' to allow stripping "revs/heads/" from the --into-name option,
but I think it is better as-is, so we use the literal value that
the user supplied.

This patch LGTM.

Thanks,
-Stolee