Start changing DiffusionCommitController to use identities

Summary: Depends on D19491.

Test Plan: Viewed some commits where the identity was mapped to a user and another that wasn't; saw the header render either a link to the user or the identity object.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D19492