gc: demonstrate failure with stale remote HEAD

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

t/t6500-gc.sh

@@ -30,4 +30,17 @@ test_expect_success 'gc -h with invalid configuration' '
 	test_i18ngrep "[Uu]sage" broken/usage
 '
 
+test_expect_failure 'gc is not aborted due to a stale symref' '
+	git init remote &&
+	(
+		cd remote &&
+		test_commit initial &&
+		git clone . ../client &&
+		git branch -m develop &&
+		cd ../client &&
+		git fetch --prune &&
+		git gc
+	)
+'
+
 test_done