Add a preference for always using unified diffs
Summary: Ref T2009. These aren't good enough to actually use so I won't land this yet, but it makes testing changes a lot easier.
Test Plan:
- Swapped setting.
- Loaded revisions.
- Saw setting respected.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T2009
Differential Revision: https://secure.phabricator.com/D11972