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