Further improve line grouping in unified views

Summary:
Ref T2009. This tweaks things a bit more to improve consecuitive groups of added and removed lines.

Generally, it gives us "old, old, old, new, new, new" intead of "old, new, old, new, old, new".

Feelin' real good about having unit tests for this stuff.

Test Plan: Unit tests, looked at diffs in web UI.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D11994