am: 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/am.c

@@ -1939,6 +1939,7 @@ next:
 	 */
 	if (!state->rebasing) {
 		am_destroy(state);
+		close_all_packs();
 		run_command_v_opt(argv_gc_auto, RUN_GIT_CMD);
 	}
 }