[PATCH v2] commit -c/-C/--amend: reset timestamp and authorship to committer with --reset-author

[PATCH v2] commit -c/-C/--amend: reset timestamp and authorship to committer with --reset-author

[Erick Mattos]

When we use -c, -C, or --amend, we are trying one of two things: using
the source as a template or modifying a commit with corrections.

When these options are used, the authorship and timestamp recorded in
the newly created commit are always taken from the original commit.
This is inconvenient when we just want to borrow the commit log message
or when our change to the code is so significant that we should take
over the authorship (with the blame for bugs we introduce, of course).

The new --reset-author option is meant to solve this need by
regenerating the timestamp and setting as the new author the committer
or the one specified by --author option.

Signed-off-by: Erick Mattos <erick.mattos@gmail.com>
---

I have remade the testing script to let it easier for people to understand and
to make it do all reasonable tests.

I have made minor message log changes and as you can see by the third paragraph
I am showing a different approach to option --author.  Please read the following
text:

--author text says: "override author for commit".

As I see, something that OVERRIDES supersedes everything else.

IMHO --author shouldn't be blocked by the new option.

Cutting --author away would make impossible for someone to force a new author
with a new timestamp in case he is templating.  As an example he can be using
the --author because he is doing a change in a computer not his own or
something alike.

So I would not wipe "author" out from the new option.

Please don't forget that I am just being a small contributor.  I am just
suggesting things.  You have the final word and if you want we can add your
small test to block it:

	if (force_author && renew_authorship)
		die("Using both --reset-author and --author does not make sense");

 Documentation/git-commit.txt |    7 ++-
 builtin-commit.c             |    9 ++-
 t/t7509-commit.sh            |  123 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 135 insertions(+), 4 deletions(-)
 create mode 100755 t/t7509-commit.sh

diff --git a/Documentation/git-commit.txt b/Documentation/git-commit.txt
index 0578a40..f89db9a 100644
--- a/Documentation/git-commit.txt
+++ b/Documentation/git-commit.txt
@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git commit' [-a | --interactive] [-s] [-v] [-u<mode>] [--amend] [--dry-run]
-	   [(-c | -C) <commit>] [-F <file> | -m <msg>]
+	   [(-c | -C) <commit>] [-F <file> | -m <msg>] [--reset-author]
 	   [--allow-empty] [--no-verify] [-e] [--author=<author>]
 	   [--cleanup=<mode>] [--] [[-i | -o ]<file>...]
 
@@ -69,6 +69,11 @@ OPTIONS
 	Like '-C', but with '-c' the editor is invoked, so that
 	the user can further edit the commit message.
 
+--reset-author::
+	When used with -C/-c/--amend options, declare that the
+	authorship of the resulting commit now belongs of the committer.
+	This also renews the author timestamp.
+
 -F <file>::
 --file=<file>::
 	Take the commit message from the given file.  Use '-' to
diff --git a/builtin-commit.c b/builtin-commit.c
index beddf01..6b51a1b 100644
--- a/builtin-commit.c
+++ b/builtin-commit.c
@@ -51,7 +51,7 @@ static const char *template_file;
 static char *edit_message, *use_message;
 static char *author_name, *author_email, *author_date;
 static int all, edit_flag, also, interactive, only, amend, signoff;
-static int quiet, verbose, no_verify, allow_empty, dry_run;
+static int quiet, verbose, no_verify, allow_empty, dry_run, renew_authorship;
 static char *untracked_files_arg;
 /*
  * The default commit message cleanup mode will remove the lines
@@ -91,8 +91,9 @@ static struct option builtin_commit_options[] = {
 	OPT_FILENAME('F', "file", &logfile, "read log from file"),
 	OPT_STRING(0, "author", &force_author, "AUTHOR", "override author for commit"),
 	OPT_CALLBACK('m', "message", &message, "MESSAGE", "specify commit message", opt_parse_m),
-	OPT_STRING('c', "reedit-message", &edit_message, "COMMIT", "reuse and edit message from specified commit "),
+	OPT_STRING('c', "reedit-message", &edit_message, "COMMIT", "reuse and edit message from specified commit"),
 	OPT_STRING('C', "reuse-message", &use_message, "COMMIT", "reuse message from specified commit"),
+	OPT_BOOLEAN(0, "reset-author", &renew_authorship, "reset timestamp and authorship to committer"),
 	OPT_BOOLEAN('s', "signoff", &signoff, "add Signed-off-by:"),
 	OPT_FILENAME('t', "template", &template_file, "use specified template file"),
 	OPT_BOOLEAN('e', "edit", &edit_flag, "force edit of commit"),
@@ -381,7 +382,7 @@ static void determine_author_info(void)
 	email = getenv("GIT_AUTHOR_EMAIL");
 	date = getenv("GIT_AUTHOR_DATE");
 
-	if (use_message) {
+	if (use_message && !renew_authorship) {
 		const char *a, *lb, *rb, *eol;
 
 		a = strstr(use_message_buffer, "\nauthor ");
@@ -780,6 +781,8 @@ static int parse_and_validate_options(int argc, const char *argv[],
 		use_message = edit_message;
 	if (amend && !use_message)
 		use_message = "HEAD";
+	if (!use_message && renew_authorship)
+		die("Option --reset-author is used only with -C/-c/--amend.");
 	if (use_message) {
 		unsigned char sha1[20];
 		static char utf8[] = "UTF-8";
diff --git a/t/t7509-commit.sh b/t/t7509-commit.sh
new file mode 100755
index 0000000..1c27de7
--- /dev/null
+++ b/t/t7509-commit.sh
@@ -0,0 +1,123 @@
+#!/bin/sh
+#
+# Copyright (c) 2009 Erick Mattos
+#
+
+test_description='git commit
+
+Tests for --reset-author option on a commit.'
+
+. ./test-lib.sh
+
+TEST_FILE=foo
+
+author_id () {
+	git cat-file -p "$1" | \
+	grep "^author" | \
+	sed -e "s/author //" -e "s/>.*/>/"
+}
+
+author_timestamp () {
+	git cat-file -p "$1" | \
+	grep "^author" | \
+	sed "s/.*> //"
+}
+
+message_body () {
+	git cat-file commit "$1" | \
+	sed -e '1,/^$/d'
+}
+
+initiate_test () {
+	test_tick
+	echo "initial" >> "$TEST_FILE"
+	git add "$TEST_FILE"
+	git commit -m "Initial Commit" --author "Frigate <flying@over.world>"
+	test_tick
+}
+
+make_files () {
+	author_id "$1" > "aid$2"
+	author_timestamp "$1" > "atime$2"
+	message_body "$1" > "message$2"
+}
+
+get_committer_id () {
+	echo "$GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL>" > aid1
+}
+
+test_expect_success '-C without --reset-author uses the author from the old commit' '
+	initiate_test &&
+	echo "Test 1" >> "$TEST_FILE" &&
+	git add "$TEST_FILE" &&
+	git commit -C HEAD &&
+	make_files HEAD^ 1 &&
+	make_files HEAD 2 &&
+	test_cmp aid1 aid2 &&
+	test_cmp atime1 atime2 &&
+	test_cmp message1 message2
+'
+
+test_expect_success '-C with --reset-author makes me the author' '
+	test_tick &&
+	echo "Test 2" >> "$TEST_FILE" &&
+	git add "$TEST_FILE" &&
+	git commit -C HEAD^ --reset-author &&
+	make_files HEAD^ 1 &&
+	make_files HEAD 2 &&
+	get_committer_id &&
+	test_cmp aid1 aid2 &&
+	test_must_fail cmp atime1 atime2 &&
+	test_cmp message1 message2
+'
+
+test_expect_success '-c without --reset-author uses the author from the old commit' '
+	initiate_test &&
+	echo "Test 3" >> "$TEST_FILE" &&
+	git add "$TEST_FILE" &&
+	EDITOR=: VISUAL=: git commit -c HEAD &&
+	make_files HEAD^ 1 &&
+	make_files HEAD 2 &&
+	test_cmp aid1 aid2 &&
+	test_cmp atime1 atime2 &&
+	test_cmp message1 message2
+'
+
+test_expect_success '-c with --reset-author makes me the author' '
+	test_tick &&
+	echo "Test 4" >> "$TEST_FILE" &&
+	git add "$TEST_FILE" &&
+	EDITOR=: VISUAL=: git commit -c HEAD^ --reset-author &&
+	make_files HEAD^ 1 &&
+	make_files HEAD 2 &&
+	get_committer_id &&
+	test_cmp aid1 aid2 &&
+	test_must_fail cmp atime1 atime2 &&
+	test_cmp message1 message2
+'
+
+test_expect_success '--amend without --reset-author uses the author from the old commit' '
+	initiate_test &&
+	make_files HEAD 2 &&
+	echo "Test 5" >> "$TEST_FILE" &&
+	git add "$TEST_FILE" &&
+	git commit -m "--amend test" --amend &&
+	make_files HEAD 1 &&
+	test_cmp aid1 aid2 &&
+	test_cmp atime1 atime2 &&
+	test_must_fail cmp message1 message2
+'
+
+test_expect_success '--amend with --reset-author makes me the author' '
+	test_tick &&
+	echo "Test 6" >> "$TEST_FILE" &&
+	git add "$TEST_FILE" &&
+	git commit -m "Changed" --amend --reset-author &&
+	make_files HEAD 2 &&
+	get_committer_id &&
+	test_cmp aid1 aid2 &&
+	test_must_fail cmp atime1 atime2 &&
+	test_must_fail cmp message1 message2
+'
+
+test_done
-- 
1.6.5.2.144.g27f8d.dirty

Re: [PATCH v2] commit -c/-C/--amend: reset timestamp and authorship to committer with --reset-author

[Junio C Hamano]

Erick Mattos <erick.mattos@gmail.com> writes:

> Cutting --author away would make impossible for someone to force a new author
> with a new timestamp in case he is templating.  As an example he can be using
> the --author because he is doing a change in a computer not his own or
> something alike.

Sorry, but I cannot help feeling a bit frustrated and mildly irritated.

I had an impression that we have already established that setting the
author with --author="Somebody Else <s@b.e>" and committing with the
current time does not make much sense from the workflow point of view long
time ago in this thread.

The mail transport might have mangled the name, and when using --amend (or
read-tree followed by commit -c), it is handy to fix the mangled name by
using --author, but in such a case you would actively want to keep the
timestamp obtained from the e-mail via either --amend or -c.

But allowing this combination, even though it might not make much sense,
is just giving extra length to the rope, so it may not be such a big deal.

I didn't feel motivated enough to read the whole thing while other patches
are in my inbox, so I instead ran diff between the previous one (without
my suggestion today) and this round.

I see that you fixed a lot of grammar in the log message of my earlier
suggestion, all of which looked very good.  Also you added a check in the
program to make sure that --renew is given only when -C/-c/--amend is
given, which is also good.  Neither of our set of tests checks this
condition, though.  IOW, we would need to add something like this at the
end of my version (adjust to --reset-author for your version):

    test_expect_success '--mine should be rejected without -c/-C/--amend' '
            git checkout Initial &&
            echo "Test 7" >>foo &&
            test_tick &&
            test_must_fail git commit -a --mine -m done
    '

I am not sure why you insist to use your version of test script and keep
changing it, though.  It looks a lot worse even only after reviewing its
early part.

 - author_id runs an extra grep that is unnecessary.  The separation of
   _id and _timestamp are unnecessary if you checked against an expected
   author ident and timestamp as a single string, i.e.

   FRIGATE='Frigate <flying@over.world>' ;# do this only once at the beginning
   ...
   git commit -C HEAD --reset-author --author="$FRIGATE" &&
   echo "author $FRIGATE $GIT_AUTHOR_TIME" >expect &&
   author_header HEAD >actual &&
   test_cmp expect actual

   This becomes irrelevant if we don't support mixing --renew and
   --author, of course.

 - message_body() now has a backslash whose sole purpose is to be an
   eyesore.

 - initiate_test() does not string the commands together with &&

I might change my mind after I take a break, review others' patches, and
spend some time on my own hacking on other topics before revisiting this
patch, but at this point I find that reviewing newer rounds of this series
has rather quickly diminishing value, and more time is being spent on
teaching shell scripting to you rather than on polishing the end result.

Sorry, but I cannot help feeling a bit frustrated and mildly irritated.
Time to take a break and attend other topics for a change.

Re: [PATCH v2] commit -c/-C/--amend: reset timestamp and authorship to committer with --reset-author

[Erick Mattos]

2009/11/4 Junio C Hamano <gitster@pobox.com>:
> Erick Mattos <erick.mattos@gmail.com> writes:
>
>> Cutting --author away would make impossible for someone to force a new author
>> with a new timestamp in case he is templating.  As an example he can be using
>> the --author because he is doing a change in a computer not his own or
>> something alike.
>
> Sorry, but I cannot help feeling a bit frustrated and mildly irritated.
>
> I had an impression that we have already established that setting the
> author with --author="Somebody Else <s@b.e>" and committing with the
> current time does not make much sense from the workflow point of view long
> time ago in this thread.
>
> The mail transport might have mangled the name, and when using --amend (or
> read-tree followed by commit -c), it is handy to fix the mangled name by
> using --author, but in such a case you would actively want to keep the
> timestamp obtained from the e-mail via either --amend or -c.
>
> But allowing this combination, even though it might not make much sense,
> is just giving extra length to the rope, so it may not be such a big deal.

I don't see a reason to be hassled by a suggestion, made because I
didn't really was confident you got it right from me from the previous
email flood.  No big deal.

> I didn't feel motivated enough to read the whole thing while other patches
> are in my inbox, so I instead ran diff between the previous one (without
> my suggestion today) and this round.

I really can't imagine all the work you have.  Probably very hard.

As we were doing before, you were saying what was wrong to you and I
was fixing it to your
demands.  So I imagine that you are doing the diffs over my versions.

I haven't got a change in that way of working.

> I see that you fixed a lot of grammar in the log message of my earlier
> suggestion, all of which looked very good.  Also you added a check in the
> program to make sure that --renew is given only when -C/-c/--amend is
> given, which is also good.  Neither of our set of tests checks this
> condition, though.  IOW, we would need to add something like this at the
> end of my version (adjust to --reset-author for your version):
>
>    test_expect_success '--mine should be rejected without -c/-C/--amend' '
>            git checkout Initial &&
>            echo "Test 7" >>foo &&
>            test_tick &&
>            test_must_fail git commit -a --mine -m done
>    '
>
> I am not sure why you insist to use your version of test script and keep
> changing it, though.  It looks a lot worse even only after reviewing its
> early part.

As I told you before I thought you were wanting me to do it.  I didn't
get a change about me working under your supervision as the coder...

I know anyone in this list is able to code those or any other change.
It is just about who is available to work at anything in particular.

>  - author_id runs an extra grep that is unnecessary.  The separation of
>   _id and _timestamp are unnecessary if you checked against an expected
>   author ident and timestamp as a single string, i.e.

author_id or author_timestamp could be changed independently and a
single string would find it corrected in any case.  The new option
ought to change both as expected in the algorithm.

>   FRIGATE='Frigate <flying@over.world>' ;# do this only once at the beginning
>   ...
>   git commit -C HEAD --reset-author --author="$FRIGATE" &&
>   echo "author $FRIGATE $GIT_AUTHOR_TIME" >expect &&
>   author_header HEAD >actual &&
>   test_cmp expect actual

If you make my script fail in any of the checks then you are going to
have "trash..." folder holding the full message log history and the
file foo with each step recorded on it with the "initials" separating
the -C, -c and --amend.  This way you can also check the differences
in the author log timestamp among cited options because the "initials"
make a barrier in between.  I made it purposely to become easier to
audit.

>   This becomes irrelevant if we don't support mixing --renew and
>   --author, of course.

It won't be supported.

>  - message_body() now has a backslash whose sole purpose is to be an
>   eyesore.

No backslash then.

>  - initiate_test() does not string the commands together with &&

It is not something difficult to add.

> I might change my mind after I take a break, review others' patches, and
> spend some time on my own hacking on other topics before revisiting this
> patch, but at this point I find that reviewing newer rounds of this series
> has rather quickly diminishing value, and more time is being spent on
> teaching shell scripting to you rather than on polishing the end result.

Now you are being impolite and then I am not saying anything back.

> Sorry, but I cannot help feeling a bit frustrated and mildly irritated.
> Time to take a break and attend other topics for a change.
>

Again:  I don't see a reason to be irritated by a suggestion...

Sorry fellow.  I just tried to help.  Let me know if you want my work anytime.

And sorry for any communication problem we had, taking in account that
I am not a native english speaker.

Best regards.

Re: [PATCH v2] commit -c/-C/--amend: reset timestamp and authorship to committer with --reset-author

[Nanako Shiraishi]

Quoting Junio C Hamano <gitster@pobox.com>

> I had an impression that we have already established that setting the
> author with --author="Somebody Else <s@b.e>" and committing with the
> current time does not make much sense from the workflow point of view long
> time ago in this thread.
> <snip>
> But allowing this combination, even though it might not make much sense,
> is just giving extra length to the rope, so it may not be such a big deal.

It may be wise to forbid a combination of options if it 
encourages mistakes or a wrong workflow, but I don't think 
using --author and --reset-author with 'git commit --amend' 
is such a case.

Imagine somebody other than you (eg. me) were the maintainer, 
and a message by Szeder was sent with a good commit log message.

 http://article.gmane.org/gmane.comp.version-control.git/132029

Then you sent a replacement patch that solves the same problem 
in a more elegant way, but without anything that is usable as the 
commit log message.

 http://article.gmane.org/gmane.comp.version-control.git/132041

If I were the maintainer, I would find it very convenient if I can 
work like this:

 % git am -s 132029   --- first I apply Szeder's version

Then I see your message. Replace the code change but use Szeder's
log message.

 % git reset --hard HEAD^
 % git am 132041   --- your version with no usable log message
 % git commit --amend -s -c @{2} --author='Junio C Hamano <...>'

> Sorry, but I cannot help feeling a bit frustrated and mildly irritated.

Don't try to be perfect and feel stressed out, and please take 
a good rest.

-- 
Nanako Shiraishi
http://ivory.ap.teacup.com/nanako3/

Re: [PATCH v2] commit -c/-C/--amend: reset timestamp and authorship to committer with --reset-author

[Junio C Hamano]

Nanako Shiraishi <nanako3@lavabit.com> writes:

> It may be wise to forbid a combination of options if it 
> encourages mistakes or a wrong workflow, but I don't think 
> using --author and --reset-author with 'git commit --amend' 
> is such a case.
>
> Imagine somebody other than you (eg. me) were the maintainer, 
> and a message by Szeder was sent with a good commit log message.
>
>  http://article.gmane.org/gmane.comp.version-control.git/132029
>
> Then you sent a replacement patch that solves the same problem 
> in a more elegant way, but without anything that is usable as the 
> commit log message.
>
>  http://article.gmane.org/gmane.comp.version-control.git/132041
>
> If I were the maintainer, I would find it very convenient if I can 
> work like this:
>
>  % git am -s 132029   --- first I apply Szeder's version
>
> Then I see your message. Replace the code change but use Szeder's
> log message.
>
>  % git reset --hard HEAD^
>  % git am 132041   --- your version with no usable log message
>  % git commit --amend -s -c @{2} --author='Junio C Hamano <...>'

Thanks.

So you commit Szeder's and then commit mine (make them independent), and
amend the log message of the latter using the message from the former, and
assign the authorship of the latter to the resulting commit?

That is a much more understandable argument than just claiming "--author
should be usable with --reset-author" without clearly stating why that
would help.  I think you forgot to add --reset-author to the last command
line, though.

But I think it is showing that --reset-author is actually suboptimal way
to solve your scenario.  In the last command in your sequence, you don't
want to add "--reset-author --author=X" but want "--reuse-only-message"
option.

And I think it makes much more sense than the alternative semantics we
came up with during this discussion.  --mine (or --reset-author) to
declare that "I am the author" was not what we wanted after all(yes, I am
guilty for suggesting it).  What we want is "I am using -C/-c/--amend and
I want to borrow only the message part from the named commit (obviously
"amend" names the HEAD commit implicitly).  Determine the authorship
information (including author timestamp) as if I didn't use that option."

Re: [PATCH v2] commit -c/-C/--amend: reset timestamp and authorship to committer with --reset-author

[Erick Mattos]

Hey, I do understand you can be very stressed.  It is a huge project.
Very important for an uncountable group of people.  A lot of demands
and argumentation from all over.

I know too that it is human nature to ask other people to agree with
them completely.  Much more when they are in charge.

So, no problem.  It was just a big surprise when I read your email.
You had been so nice to me until that moment...

But let's keep talking about code.  I am not a big fan of human nature subjects.

Although I have to be more personal for just a little.  I want to show
you my way of seeing things:

I love defaults!  A command or an option already set for the most
common scenario... It's wonderful.

But I like to have full control of any tool I use.  If I want to do
any bizarre thing nobody has
ever thought about... I think I should be able to.  Without any hacking.

I can hammer a nail with a wrench.  But I would prefer a hammer for that.

I think your suggestions which changed the path of this intended
function since the beginning were very good for a default.  So I think
--reset-author did it.  Normally 95% of the time its behavior is what
people will be needing.

But cutting off a remote possibility for no heavy and unbearable
reason imho makes features incomplete.  That is why I had suggested
not cutting off --author functionality when using --reset-author.

I did not try to conceive all possible uses for this combination but I
knew someone could find some.  I have told you a simple case just to
picture some figures.  Nanako showed you a case you agreed.  Thanks
Nanako.

I was not defying your judgment or showing lack of respect to you.  My
text after "---" was very clear about that.  Thank you again Nanako
for showing me the importance of this little text.

About scripting abilities: I don't see a way to compare scripting
"levels".  Scripts are so easy that you just know or not.

Different approaches could be compared.  At start I really did not get
the use of the "t" folder tests.  I thought it was just to show
functionalities.

Nanako in her critics made me understand within her speach the
importance of those tests.  Then you clarified it much more later.  So
I got those informations and made another script trying to test
--reset-author completely.  Separating every bit of data that could
show a malfunctioning.  And taking also the care of letting auditing
more reliable and informational.

So I accepted your rough saying about "teaching" as an explosion of stress.

I have to tell that our work-flow on that time was: you demanded and I
made a change.  The script you added was an example to me under this
work-flow.

I am not a kid and I have a real and busy life but I do think spending
time sharing some changes I use to improve something which I value is
not a lost time.  As you can see by the time I had sent the emails, I
was doing them overtime.

So I would like to make clear that I did and do want to help as much
as I can.  If it is not possible to use my work then just know you and
every free software coder has a big fan in me.  I will be transmitting
good energies to you all in any case.

No hard feelings.  :-)

I hope you can continue doing the wonderful work you have been doing
for a very long future.

Best regards.

2009/11/5 Junio C Hamano <gitster@pobox.com>:
> Nanako Shiraishi <nanako3@lavabit.com> writes:
>
>> It may be wise to forbid a combination of options if it
>> encourages mistakes or a wrong workflow, but I don't think
>> using --author and --reset-author with 'git commit --amend'
>> is such a case.
>>
>> Imagine somebody other than you (eg. me) were the maintainer,
>> and a message by Szeder was sent with a good commit log message.
>>
>>  http://article.gmane.org/gmane.comp.version-control.git/132029
>>
>> Then you sent a replacement patch that solves the same problem
>> in a more elegant way, but without anything that is usable as the
>> commit log message.
>>
>>  http://article.gmane.org/gmane.comp.version-control.git/132041
>>
>> If I were the maintainer, I would find it very convenient if I can
>> work like this:
>>
>>  % git am -s 132029   --- first I apply Szeder's version
>>
>> Then I see your message. Replace the code change but use Szeder's
>> log message.
>>
>>  % git reset --hard HEAD^
>>  % git am 132041   --- your version with no usable log message
>>  % git commit --amend -s -c @{2} --author='Junio C Hamano <...>'
>
> Thanks.
>
> So you commit Szeder's and then commit mine (make them independent), and
> amend the log message of the latter using the message from the former, and
> assign the authorship of the latter to the resulting commit?
>
> That is a much more understandable argument than just claiming "--author
> should be usable with --reset-author" without clearly stating why that
> would help.  I think you forgot to add --reset-author to the last command
> line, though.
>
> But I think it is showing that --reset-author is actually suboptimal way
> to solve your scenario.  In the last command in your sequence, you don't
> want to add "--reset-author --author=X" but want "--reuse-only-message"
> option.
>
> And I think it makes much more sense than the alternative semantics we
> came up with during this discussion.  --mine (or --reset-author) to
> declare that "I am the author" was not what we wanted after all(yes, I am
> guilty for suggesting it).  What we want is "I am using -C/-c/--amend and
> I want to borrow only the message part from the named commit (obviously
> "amend" names the HEAD commit implicitly).  Determine the authorship
> information (including author timestamp) as if I didn't use that option."
>