[PATCH v4] pull: warn if the user didn't say whether to rebase or to merge

[Alex Henrie <alexhenrie24@gmail.com>]

Often novice Git users forget to say "pull --rebase" and end up with an
unnecessary merge from upstream. What they usually want is either "pull
--rebase" in the simpler cases, or "pull --ff-only" to update the copy
of main integration branches, and rebase their work separately. The
pull.rebase configuration variable exists to help them in the simpler
cases, but there is no mechanism to make these users aware of it.

Issue a warning message when no --[no-]rebase option from the command
line and no pull.rebase configuration variable is given. This will
inconvenience those who never want to "pull --rebase", who haven't had
to do anything special, but the cost of the inconvenience is paid only
once per user, which should be a reasonable cost to help a number of new
users.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
---
v4:
- Revise warning message based on Junio's feedback
- Consistently wrap warning lines to 75 characters for easy viewing in
PO files
- Fix test failures
---
 builtin/pull.c          | 15 +++++++++++++++
 t/t5521-pull-options.sh |  3 ++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/builtin/pull.c b/builtin/pull.c
index 3e624d1e00..351b933c4d 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -327,6 +327,21 @@ static enum rebase_type config_get_rebase(void)
 	if (!git_config_get_value("pull.rebase", &value))
 		return parse_config_rebase("pull.rebase", value, 1);
 
+	if (!opt_ff || strcmp(opt_ff, "--ff-only")) {
+		warning(_("Pulling without specifying how to reconcile divergent branches is\n"
+			"discouraged. You can squelch this message by running one of the following\n"
+			"commands sometime before your next pull:\n"
+			"\n"
+			"  git config pull.rebase false  # merge (the default strategy)\n"
+			"  git config pull.rebase true   # rebase\n"
+			"  git config pull.ff only       # fast-forward only\n"
+			"\n"
+			"You can replace \"git config\" with \"git config --global\" to set a default\n"
+			"preference for all repositories. You can also pass --rebase, --no-rebase,\n"
+			"or --ff-only on the command line to override the configured default per\n"
+			"invocation.\n"));
+	}
+
 	return REBASE_FALSE;
 }
 
diff --git a/t/t5521-pull-options.sh b/t/t5521-pull-options.sh
index ccde8ba491..6e890ec936 100755
--- a/t/t5521-pull-options.sh
+++ b/t/t5521-pull-options.sh
@@ -8,7 +8,8 @@ test_expect_success 'setup' '
 	mkdir parent &&
 	(cd parent && git init &&
 	 echo one >file && git add file &&
-	 git commit -m one)
+	 git commit -m one) &&
+	git config pull.rebase false
 '
 
 test_expect_success 'git pull -q' '
-- 
2.25.1