ls-files -h: show usage even with corrupt index

Part of a campaign to avoid git <command> -h being distracted by
access to the repository.  A caller hoping to use "git ls-files"
with an alternate index as part of a repair operation may well use
"git ls-files -h" to show usage while planning it out.

[jn: with rewritten log message and tests]

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/ls-files.c

@@ -530,6 +530,9 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 		OPT_END()
 	};
 
+	if (argc == 2 && !strcmp(argv[1], "-h"))
+		usage_with_options(ls_files_usage, builtin_ls_files_options);
+
 	memset(&dir, 0, sizeof(dir));
 	prefix = cmd_prefix;
 	if (prefix)