Make CommitController more flexible about handling URIs

Summary:
Ref T4245. This adds support for both ID-based and callsign-based routes, although the ID-based routes don't occur anywhere.

Also moves toward simplifying the DiffusionRequest stuff.

Test Plan: Visited normal callsign-based commit pages; visited new ID-based commit pages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14940