Convert "Diff Details" tabs to PHUITabGroup

Summary:
Ref T10628. Switch this to be nicer and more modern.

• When there's only one tab, add an option to hide it.

Test Plan:

• Viewed normal revisions (no tabs).
• Viewed X vs Y revisions (two tabs, rightmost tab selected by default).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10628

Differential Revision: https://secure.phabricator.com/D16206