submodule--helper: show usage for "-h"

Normal users shouldn't ever call submodule--helper, but it
doesn't hurt to give them a normal usage message if they try
"-h".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -1222,9 +1222,8 @@ static struct cmd_struct commands[] = {
 int cmd_submodule__helper(int argc, const char **argv, const char *prefix)
 {
 	int i;
-	if (argc < 2)
-		die(_("submodule--helper subcommand must be "
-		      "called with a subcommand"));
+	if (argc < 2 || !strcmp(argv[1], "-h"))
+		usage("git submodule--helper <command>");
 
 	for (i = 0; i < ARRAY_SIZE(commands); i++) {
 		if (!strcmp(argv[1], commands[i].cmd)) {