path.c: move some code out of strbuf_git_path_submodule()

refs is learning to avoid path rewriting that is done by
strbuf_git_path_submodule(). Factor out this code so it could be reused
by refs_* functions.

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

submodule.c

@@ -1596,3 +1596,34 @@ const char *get_superproject_working_tree(void)
 
 	return ret;
 }
+
+int submodule_to_gitdir(struct strbuf *buf, const char *submodule)
+{
+	const struct submodule *sub;
+	const char *git_dir;
+	int ret = 0;
+
+	strbuf_reset(buf);
+	strbuf_addstr(buf, submodule);
+	strbuf_complete(buf, '/');
+	strbuf_addstr(buf, ".git");
+
+	git_dir = read_gitfile(buf->buf);
+	if (git_dir) {
+		strbuf_reset(buf);
+		strbuf_addstr(buf, git_dir);
+	}
+	if (!is_git_directory(buf->buf)) {
+		gitmodules_config();
+		sub = submodule_from_path(null_sha1, submodule);
+		if (!sub) {
+			ret = -1;
+			goto cleanup;
+		}
+		strbuf_reset(buf);
+		strbuf_git_path(buf, "%s/%s", "modules", sub->name);
+	}
+
+cleanup:
+	return ret;
+}