Let 'git <command> -h' show usage without a git dir

There is no need for "git <command> -h" to depend on being inside
a repository.

Reported by Gerfried Fuchs through http://bugs.debian.org/462557

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

index-pack.c

@@ -882,6 +882,9 @@ int main(int argc, char **argv)
 
 	git_extract_argv0_path(argv[0]);
 
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage(index_pack_usage);
+
 	/*
 	 * We wish to read the repository's config file if any, and
 	 * for that it is necessary to call setup_git_directory_gently().