Fix call to getRefType on non-object in PhabricatorRepositoryEngine.php

Summary:
Error:
  Fatal error: Call to a member function getRefType() on a non-object in /opt/phabricator/phabricator/src/applications/repository/engine/PhabricatorRepositoryRefEngine.php on line 197

Test Plan: No more error in daemon.log afterwards

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8278