merge -h: show usage even with corrupt index

Part of a campaign to make sure "git <command> -h" works correctly
when run from distractingly bad repositories.

[jn: with rewritten log message and tests]
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -909,6 +909,8 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	const char *best_strategy = NULL, *wt_strategy = NULL;
 	struct commit_list **remotes = &remoteheads;
 
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(builtin_merge_usage, builtin_merge_options);
 	if (read_cache_unmerged()) {
 		die_resolve_conflict("merge");
 	}

t/t7600-merge.sh

@@ -144,6 +144,17 @@ test_expect_success 'test option parsing' '
 	test_must_fail git merge
 '
 
+test_expect_success 'merge -h with invalid index' '
+	mkdir broken &&
+	(
+		cd broken &&
+		git init &&
+		>.git/index &&
+		test_expect_code 129 git merge -h 2>usage
+	) &&
+	grep "[Uu]sage: git merge" broken/usage
+'
+
 test_expect_success 'reject non-strategy with a git-merge-foo name' '
 	test_must_fail git merge -s index c1
 '