branch -h: show usage even in an invalid repository

There is no need for "git branch -h" to try to access a repository.

In the spirit of v1.6.6-rc0~34^2~3 (Let 'git <command> -h' show usage
without a git dir, 2009-11-09).  This brings git one step closer to
passing the following (automatically verifiable) test:

 Before any repository access (aside from git_config()), a
 function from the setup_git_directory_* family has been run

and thus one step closer to being able to use an automatic repository
access checker.

[jn: simplified; new commit message, test]
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/branch.c

@@ -667,6 +667,9 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		OPT_END(),
 	};
 
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(builtin_branch_usage, options);
+
 	git_config(git_branch_config, NULL);
 
 	if (branch_use_color == -1)

t/t3200-branch.sh

@@ -26,6 +26,17 @@ test_expect_success \
      ! test -f .git/refs/heads/--help
 '
 
+test_expect_success 'branch -h in broken repository' '
+	mkdir broken &&
+	(
+		cd broken &&
+		git init &&
+		>.git/refs/heads/master &&
+		test_expect_code 129 git branch -h >usage 2>&1
+	) &&
+	grep "[Uu]sage" broken/usage
+'
+
 test_expect_success \
     'git branch abc should create a branch' \
     'git branch abc && test -f .git/refs/heads/abc'