[PATCH v3] 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>
---
v3:
- Fix segfault if opt_ff is NULL
- Don't say that the behavior will change in a future version of Git
(that hasn't been decided yet)
- Mention that pull.ff=only is also an option
- Mention git config's --global option
- Make the warning message longer and more clear in general
---
 builtin/pull.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/builtin/pull.c b/builtin/pull.c
index 3e624d1e00..a93ee489f6 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -327,6 +327,20 @@ 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\n"
+			"is discouraged. You can squelch this message by running one of the\n"
+			"following 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. Alternatively, you can pass --rebase,\n"
+			"--no-rebase, or --ff-only on the command line every time you pull.\n"));
+	}
+
 	return REBASE_FALSE;
 }
 
-- 
2.25.1