merge: release pack files before garbage-collecting

Before auto-gc'ing, we need to make sure that the pack files are
released in case they need to be repacked and garbage-collected.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -404,6 +404,7 @@ static void finish(struct commit *head_commit,
 			 * We ignore errors in 'gc --auto', since the
 			 * user should see them.
 			 */
+			close_all_packs();
 			run_command_v_opt(argv_gc_auto, RUN_GIT_CMD);
 		}
 	}