submodule refactor: use strbuf_git_path_submodule() in add_submodule_odb()

Functions which directly operate submodule's object database do not
handle the case when the submodule is linked worktree (which are
introduced in c7b3a3d2fe). Instead of fixing the path calculation use
already existing strbuf_git_path_submodule() function without changing
overall behaviour. Then it will be possible to modify only that function
whenever we need to change real location of submodule's repository
content.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -122,15 +122,8 @@ static int add_submodule_odb(const char *path)
 	struct strbuf objects_directory = STRBUF_INIT;
 	struct alternate_object_database *alt_odb;
 	int ret = 0;
-	const char *git_dir;
 
-	strbuf_addf(&objects_directory, "%s/.git", path);
-	git_dir = read_gitfile(objects_directory.buf);
-	if (git_dir) {
-		strbuf_reset(&objects_directory);
-		strbuf_addstr(&objects_directory, git_dir);
-	}
-	strbuf_addstr(&objects_directory, "/objects/");
+	strbuf_git_path_submodule(&objects_directory, path, "objects/");
 	if (!is_directory(objects_directory.buf)) {
 		ret = -1;
 		goto done;