refs.c: remove fallback-to-main-store code get_submodule_ref_store()

At this state, there are three get_submodule_ref_store() callers:

 - for_each_remote_ref_submodule()
 - handle_revision_pseudo_opt()
 - resolve_gitlink_ref()

The first two deal explicitly with submodules (and we should never fall
back to the main ref store as a result). They are only called from
submodule.c:

 - find_first_merges()
 - submodule_needs_pushing()
 - push_submodule()

The last one, as its name implies, deals only with submodules too, and
the "submodule" (path) argument must be a non-NULL, non-empty string.

So, this "if NULL or empty string" code block should never ever
trigger. And it's wrong to fall back to the main ref store
anyway. Delete it.

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

refs.c

@@ -1587,6 +1587,9 @@ struct ref_store *get_submodule_ref_store(const char *submodule)
 	char *to_free = NULL;
 	size_t len;
 
+	if (!submodule)
+		return NULL;
+
 	if (submodule) {
 		len = strlen(submodule);
 		while (len && is_dir_sep(submodule[len - 1]))
@@ -1595,14 +1598,6 @@ struct ref_store *get_submodule_ref_store(const char *submodule)
 			return NULL;
 	}
 
-	if (!submodule || !*submodule) {
-		/*
-		 * FIXME: This case is ideally not allowed. But that
-		 * can't happen until we clean up all the callers.
-		 */
-		return get_main_ref_store();
-	}
-
 	if (submodule[len])
 		/* We need to strip off one or more trailing slashes */
 		submodule = to_free = xmemdupz(submodule, len);