Unhighlight lines modified in rebase

Summary:
Diff of diffs display changes between new versions of file.
This is bad after rebase because there can be many unrelated changes so it is hard to spot the real change.

This diff unhighlights the lines that were added or removed in rebase.
The changes are still visible (they can be sometimes relevant) but very subtle.

Test Plan:

1. Add, change and delete line. Display diff.
2. Add and change some lines in parent. Rebase. Display diff. Display diff of diff.
3. Change and add some lines. Display diff. Display diff to first diff. Display diff to second diff.

Reviewers: epriestley, jungejason

Reviewed By: jungejason

CC: jungejason, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2761