R1:b2d280ff5142

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"…