Allow DocumentEngine to elect into diff construction
Ref T13425. Allow DocumentEngines to claim they can produce diffs. If both sides of a change can be diffed by the same document engine and it can produce diffs, have it diff them.
This has no impact on runtime behavior because no upstream engine elects into diff generation yet.
Test Plan: Loaded some revisions, nothing broke.
Maniphest Tasks: T13425
Differential Revision: https://secure.phabricator.com/D20830