ref_transaction_commit(): check for valid `transaction->state`

Move the check that `transaction->state` is valid from
`files_transaction_commit()` to `ref_transaction_commit()`, where
other future reference backends can benefit from it as well.

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

refs.c

@@ -1694,6 +1694,18 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 {
 	struct ref_store *refs = transaction->ref_store;
 
+	switch (transaction->state) {
+	case REF_TRANSACTION_OPEN:
+		/* Good. */
+		break;
+	case REF_TRANSACTION_CLOSED:
+		die("BUG: prepare called on a closed reference transaction");
+		break;
+	default:
+		die("BUG: unexpected reference transaction state");
+		break;
+	}
+
 	if (getenv(GIT_QUARANTINE_ENVIRONMENT)) {
 		strbuf_addstr(err,
 			      _("ref updates forbidden inside quarantine environment"));

refs/files-backend.c

@@ -2874,9 +2874,6 @@ static int files_transaction_commit(struct ref_store *ref_store,
 
 	assert(err);
 
-	if (transaction->state != REF_TRANSACTION_OPEN)
-		die("BUG: commit called for transaction that is not open");
-
 	if (!transaction->nr)
 		goto cleanup;