ref_transaction_update(): die on disallowed flags

Callers shouldn't be passing disallowed flags into
`ref_transaction_update()`. So instead of masking them off, treat it
as a bug if any are set.

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

Author: mhagger

refs.c

@@ -940,7 +940,8 @@ int ref_transaction_update(struct ref_transaction *transaction,
 		return -1;
 	}
 
-	flags &= REF_TRANSACTION_UPDATE_ALLOWED_FLAGS;
+	if (flags & ~REF_TRANSACTION_UPDATE_ALLOWED_FLAGS)
+		BUG("illegal flags 0x%x passed to ref_transaction_update()", flags);
 
 	flags |= (new_oid ? REF_HAVE_NEW : 0) | (old_oid ? REF_HAVE_OLD : 0);