Update DiffusionLastModifiedController to use identities

Summary: Ref T12164. Updates another controller to use identities.

Test Plan:
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,
);

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12164

Differential Revision: https://secure.phabricator.com/D19580