R1:932d829af339

Improve behavior of inline comment highlight reticle for block diffs

Summary:
Depends on D20834. Ref T13425. After the change from "th" to "td" for accessibility, the algorithm picks which cells it should highlight slightly improperly (it picks too many cells since it can no longer find the line numbers).

Ideally, it would probably highlight //only// the source content, but there isn't an easy way to do this right now. Settle for an incremental improvement for the moment.

Test Plan: Hovered over line…