packed_delete_refs(): implement method

Implement `packed_delete_refs()` using a reference transaction. This
means that `files_delete_refs()` can use `refs_delete_refs()` instead
of `repack_without_refs()` to delete any packed references, decreasing
the coupling between the classes.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -1157,7 +1157,7 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
 		goto error;
 
-	if (repack_without_refs(refs->packed_ref_store, refnames, &err)) {
+	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
 		packed_refs_unlock(refs->packed_ref_store);
 		goto error;
 	}

refs/packed-backend.c

@@ -1086,7 +1086,50 @@ static int packed_initial_transaction_commit(struct ref_store *ref_store,
 static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
 			     struct string_list *refnames, unsigned int flags)
 {
-	die("BUG: not implemented yet");
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct strbuf err = STRBUF_INIT;
+	struct ref_transaction *transaction;
+	struct string_list_item *item;
+	int ret;
+
+	(void)refs; /* We need the check above, but don't use the variable */
+
+	if (!refnames->nr)
+		return 0;
+
+	/*
+	 * Since we don't check the references' old_oids, the
+	 * individual updates can't fail, so we can pack all of the
+	 * updates into a single transaction.
+	 */
+
+	transaction = ref_store_transaction_begin(ref_store, &err);
+	if (!transaction)
+		return -1;
+
+	for_each_string_list_item(item, refnames) {
+		if (ref_transaction_delete(transaction, item->string, NULL,
+					   flags, msg, &err)) {
+			warning(_("could not delete reference %s: %s"),
+				item->string, err.buf);
+			strbuf_reset(&err);
+		}
+	}
+
+	ret = ref_transaction_commit(transaction, &err);
+
+	if (ret) {
+		if (refnames->nr == 1)
+			error(_("could not delete reference %s: %s"),
+			      refnames->items[0].string, err.buf);
+		else
+			error(_("could not delete references: %s"), err.buf);
+	}
+
+	ref_transaction_free(transaction);
+	strbuf_release(&err);
+	return ret;
 }
 
 static int packed_pack_refs(struct ref_store *ref_store, unsigned int flags)