R1:7acda7e94e60

Truncate package names in diff table of contents views

Summary:
Ref T13151. See PHI654. Depends on D19477. If you have long package names, the table of contents (e.g., in Differential) can end up expanding to be gigantic.

Getting tables to behave nicely is hard (or, at least, I can't figure it out after spending a decent amount of time on it; see also `AphrontTableView::renderSingleDisplayLine()`). I tried a bunch of things and Googled for a bit but didn't make any progress on finding a CSS solution.