commit: run git gc --auto just before the post-commit hook

Change the behavior of git-commit back to what it was back in
d4bb43ee27 ("Invoke "git gc --auto" from commit, merge, am and
rebase.", 2007-09-05) when it was git-commit.sh.

Shortly afterwards in f5bbc3225c ("Port git commit to C.", 2007-11-08)
when it was ported to C, the "git gc --auto" invocation went away.

Since that unintended regression, git gc --auto only ran for git-am,
git-merge, git-fetch, and git-receive-pack.  It was possible to
write a script that would "git commit" a lot of data locally, and gc
would never run.

One such repository that was locally committing generated zone file
changes had grown to a size of ~60GB before a daily cronjob was added
to "git gc", bringing it down to less than 1GB. This will make such
cases work without intervention.

I think fixing such pathological cases where the repository will grow
forever is a worthwhile trade-off for spending a couple of
milliseconds calling "git gc --auto" (in the common cases where it
doesn't do anything).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1615,6 +1615,7 @@ int run_commit_hook(int editor_is_used, const char *index_file, const char *name
 
 int cmd_commit(int argc, const char **argv, const char *prefix)
 {
+	const char *argv_gc_auto[] = {"gc", "--auto", NULL};
 	static struct wt_status s;
 	static struct option builtin_commit_options[] = {
 		OPT__QUIET(&quiet, N_("suppress summary after successful commit")),
@@ -1835,6 +1836,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		     "not exceeded, and then \"git reset HEAD\" to recover."));
 
 	rerere(0);
+	run_command_v_opt(argv_gc_auto, RUN_GIT_CMD);
 	run_commit_hook(use_editor, get_index_file(), "post-commit", NULL);
 	if (amend && !no_post_rewrite) {
 		struct notes_rewrite_cfg *cfg;