Make the revision graph view more flexible

Summary:
Ref T4788. This separates the revision graph view into a base class with core logic and a revision class with Differential-specific logic, so I can subclass it in Maniphest, etc., and try using it in other applications to show similar graphs.

Not sure if we'll stick with it, but even if we don't this makes the code a bit cleaner and gets custom rendering logic out of the RevisionViewController, which is nice.

Test Plan: Viewed revisions, saw the stack UI completely unchanged.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4788

Differential Revision: https://secure.phabricator.com/D16213