Update DiffusionLastModifiedController to use identities
Summary: Ref T12164. Updates another controller to use identities.
Pretty ad-hoc, but loaded the main pages of several different repos with and without repo identities. I'm not totally convinced the author from this data structure is actually being used:
$return = array( 'commit' => $modified, 'date' => $date, 'author' => $author, 'details' => $details, );
Reviewed By: epriestley
Maniphest Tasks: T12164
Differential Revision: https://secure.phabricator.com/D19580