refs: rename lookup_ref_store() to lookup_submodule_ref_store()

With get_main_ref_store() being used inside get_ref_store(),
lookup_ref_store() is only used for submodule code path. Rename to
reflect that and delete dead code.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs.c

@@ -1395,17 +1395,13 @@ static struct ref_store *main_ref_store;
 static struct hashmap submodule_ref_stores;
 
 /*
- * Return the ref_store instance for the specified submodule (or the
- * main repository if submodule is NULL). If that ref_store hasn't
- * been initialized yet, return NULL.
+ * Return the ref_store instance for the specified submodule. If that
+ * ref_store hasn't been initialized yet, return NULL.
  */
-static struct ref_store *lookup_ref_store(const char *submodule)
+static struct ref_store *lookup_submodule_ref_store(const char *submodule)
 {
 	struct submodule_hash_entry *entry;
 
-	if (!submodule)
-		return main_ref_store;
-
 	if (!submodule_ref_stores.tablesize)
 		/* It's initialized on demand in register_ref_store(). */
 		return NULL;
@@ -1471,7 +1467,7 @@ struct ref_store *get_ref_store(const char *submodule)
 	if (!submodule || !*submodule) {
 		return get_main_ref_store();
 	} else {
-		refs = lookup_ref_store(submodule);
+		refs = lookup_submodule_ref_store(submodule);
 
 		if (!refs) {
 			struct strbuf submodule_sb = STRBUF_INIT;
@@ -1482,7 +1478,6 @@ struct ref_store *get_ref_store(const char *submodule)
 			strbuf_release(&submodule_sb);
 		}
 	}
-
 	return refs;
 }