refs: new transaction related ref-store api

The transaction struct now takes a ref store at creation and will
operate on that ref store alone.

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

refs/refs-internal.h

@@ -200,6 +200,7 @@ enum ref_transaction_state {
  * as atomically as possible.  This structure is opaque to callers.
  */
 struct ref_transaction {
+	struct ref_store *ref_store;
 	struct ref_update **updates;
 	size_t alloc;
 	size_t nr;