[PATCH v2] 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>
---
v2:
- Use the commit message that Junio suggested
- Drop "!= 0" from the if condition
- Mention in the warning that --ff-only will also suppress the warning
---
 builtin/pull.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/builtin/pull.c b/builtin/pull.c
index 3e624d1e00..96512a4b27 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -327,6 +327,13 @@ 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 (strcmp(opt_ff, "--ff-only")) {
+		warning(_("Pulling without specifying whether to rebase or to merge is discouraged\n"
+			"and will be disallowed in a future Git release.\n"
+			"Next time, run `git config pull.rebase (true|false)` first\n"
+			"or pass --rebase, --no-rebase, or --ff-only on the command line.\n"));
+	}
+
 	return REBASE_FALSE;
 }
 
-- 
2.25.1