submodule--helper update-clone: check for --filter and --init

"git submodule update --filter" also requires the "--init" option. Teach
update-clone to do this usage check in C and remove the check from
git-submodule.sh.

In addition, change update-clone's usage string so that it teaches users
about "git submodule update" instead of "git submodule--helper
update-clone" (the string is copied from git-submodule.sh). This should
be more helpful to users since they don't invoke update-clone directly.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -2545,7 +2545,12 @@ static int update_clone(int argc, const char **argv, const char *prefix)
 	};
 
 	const char *const git_submodule_helper_usage[] = {
-		N_("git submodule--helper update-clone [--prefix=<path>] [<path>...]"),
+		N_("git submodule [--quiet] update"
+		" [--init [--filter=<filter-spec>]] [--remote]"
+		" [-N|--no-fetch] [-f|--force]"
+		" [--checkout|--merge|--rebase]"
+		" [--[no-]recommend-shallow] [--reference <repository>]"
+		" [--recursive] [--[no-]single-branch] [--] [<path>...]"),
 		NULL
 	};
 	suc.prefix = prefix;
@@ -2556,6 +2561,19 @@ static int update_clone(int argc, const char **argv, const char *prefix)
 	memset(&filter_options, 0, sizeof(filter_options));
 	argc = parse_options(argc, argv, prefix, module_update_clone_options,
 			     git_submodule_helper_usage, 0);
+
+	if (filter_options.choice && !suc.init) {
+		/*
+		 * NEEDSWORK: Don't use usage_with_options() because the
+		 * usage string is for "git submodule update", but the
+		 * options are for "git submodule--helper update-clone".
+		 *
+		 * This will no longer be an issue when "update-clone"
+		 * is replaced by "git submodule--helper update".
+		 */
+		usage(git_submodule_helper_usage[0]);
+	}
+
 	suc.filter_options = &filter_options;
 
 	if (update)

git-submodule.sh

@@ -356,11 +356,6 @@ cmd_update()
 		shift
 	done
 
-	if test -n "$filter" && test "$init" != "1"
-	then
-		usage
-	fi
-
 	{
 	git ${wt_prefix:+-C "$wt_prefix"} submodule--helper update-clone \
 		${GIT_QUIET:+--quiet} \