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