Handle the case when a repository was deleted

Summary:
when a repository was deleted,
PhabricatorObjectHandleData::loadHandles() is throwing exception because
it assumes that the repository for the commit exists.

Test Plan: try an revision whose repo was deleted and it renders.
Reviewed By: epriestley
Reviewers: epriestley, andrewjcg
CC: aran, epriestley
Differential Revision: 576