Prefer left-side for old comments in new-vs-new diff of diffs

Summary: Ref T7447. Ref T7870.

Test Plan:
Before:

{F377691}

After:

{F377692}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7870, T7447

Differential Revision: https://secure.phabricator.com/D12489