gc --auto: release pack files before auto packing

Teach gc --auto to release pack files before auto packing the repository
to prevent failures when removing them.

Also teach the test 'fetching with auto-gc does not lock up' to complain
when it is no longer triggering an auto packing of the repository.

Fixes https://github.com/git-for-windows/git/issues/500

Signed-off-by: Kim Gybels <kgybels@infogroep.be>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/gc.c

@@ -612,6 +612,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 		return -1;
 
 	if (!repository_format_precious_objects) {
+		close_all_packs(the_repository->objects);
 		if (run_command_v_opt(repack.argv, RUN_GIT_CMD))
 			return error(FAILED_RUN, repack.argv[0]);
 

t/t5510-fetch.sh

@@ -828,9 +828,11 @@ test_expect_success 'fetching with auto-gc does not lock up' '
 	test_commit test2 &&
 	(
 		cd auto-gc &&
+		git config fetch.unpackLimit 1 &&
 		git config gc.autoPackLimit 1 &&
 		git config gc.autoDetach false &&
 		GIT_ASK_YESNO="$D/askyesno" git fetch >fetch.out 2>&1 &&
+		test_i18ngrep "Auto packing the repository" fetch.out &&
 		! grep "Should I try again" fetch.out
 	)
 '