R1:5c4c593af325

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…