R1:a238f6a7598a

Implement rough content-aware inline adjustment rules for ghosts

Summary:
Ref T7447. Fixes T7600. This likely needs significant adjustment, but implements content-aware comment porting for line changes.

Specifically, this moves lines around to adjust their position considering added and removed lines between the diffs and across rebases.

It does not try to do any actual content (line against line) matching.

Test Plan:
- Unit tests.
- Poking around in the web UI seems to generate mostly reasonable…