Invoke "git gc --auto" from commit, merge, am and rebase.

The point of auto gc is to pack new objects created in loose
format, so a good rule of thumb is where we do update-ref after
creating a new commit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-commit.sh

@@ -652,6 +652,7 @@ git rerere
 
 if test "$ret" = 0
 then
+	git gc --auto
 	if test -x "$GIT_DIR"/hooks/post-commit
 	then
 		"$GIT_DIR"/hooks/post-commit