submodule--helper: add remote-branch helper

In a later patch we want to enhance the logic for the branch selection.
Rewrite the current logic to be in C, so we can directly use C when
we enhance the logic.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -899,6 +899,39 @@ static int resolve_relative_path(int argc, const char **argv, const char *prefix
 	return 0;
 }
 
+static const char *remote_submodule_branch(const char *path)
+{
+	const struct submodule *sub;
+	gitmodules_config();
+	git_config(submodule_config, NULL);
+
+	sub = submodule_from_path(null_sha1, path);
+	if (!sub)
+		return NULL;
+
+	if (!sub->branch)
+		return "master";
+
+	return sub->branch;
+}
+
+static int resolve_remote_submodule_branch(int argc, const char **argv,
+		const char *prefix)
+{
+	const char *ret;
+	struct strbuf sb = STRBUF_INIT;
+	if (argc != 2)
+		die("submodule--helper remote-branch takes exactly one arguments, got %d", argc);
+
+	ret = remote_submodule_branch(argv[1]);
+	if (!ret)
+		die("submodule %s doesn't exist", argv[1]);
+
+	printf("%s", ret);
+	strbuf_release(&sb);
+	return 0;
+}
+
 struct cmd_struct {
 	const char *cmd;
 	int (*fn)(int, const char **, const char *);
@@ -912,7 +945,8 @@ static struct cmd_struct commands[] = {
 	{"relative-path", resolve_relative_path},
 	{"resolve-relative-url", resolve_relative_url},
 	{"resolve-relative-url-test", resolve_relative_url_test},
-	{"init", module_init}
+	{"init", module_init},
+	{"remote-branch", resolve_remote_submodule_branch}
 };
 
 int cmd_submodule__helper(int argc, const char **argv, const char *prefix)

git-submodule.sh

@@ -614,7 +614,7 @@ cmd_update()
 
 		if test -n "$remote"
 		then
-			branch=$(get_submodule_config "$name" branch master)
+			branch=$(git submodule--helper remote-branch "$sm_path")
 			if test -z "$nofetch"
 			then
 				# Fetch remote before determining tracking $sha1