Port comments through time and space in the common/best case

Summary:
Ref T7447. This ports comments forward and backward in the best case:

• The old comment is on a changeset with the same filename.
• The old and new files are pretty much the same, line-for-line.

This will fail to port a lot of comments around and probably port a lot of comments into goofy places. We can see how bad it is in practice.

Errata:

• Design is me cobbling something together, probably worth tweaking.
• "Old Comment" should, at a minimum, say "Newer Comment" sometimes, or we should come up with some better name for this stuff.

Test Plan:

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: johnny-bit, yelirekim, epriestley

Maniphest Tasks: T7447

Differential Revision: https://secure.phabricator.com/D12484