Fix another issue with line order on unified diffs

Summary: This improves some cases with interleaved added and removed lines, and adds test coverage.

Test Plan:

• Added and executed unit tests.
• Viewed raw diff and saw sensible/expected output.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11992