R1:b3d1ecebc7f8

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