Use more consistent diff coloration in unified diffs

Summary:
Ref T13151. See PHI701. Unified diffs are currently missing the logic to apply the "old-full" and "new-full" classes, which results in a too-light coloration for fully added or removed lines.

Make this logic consistent with the two-up renderer so we use the same colors in both.

Test Plan: Viewed diffs and swapped between 1-up and 2-up renderers, now saw the same coloration.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19482