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-merge.sh

@@ -82,6 +82,7 @@ finish () {
 			;;
 		*)
 			git update-ref -m "$rlogm" HEAD "$1" "$head" || exit 1
+			git gc --auto
 			;;
 		esac
 		;;