Improve string truncation behavior for strings truncated in a very long word

Summary:
Ref T13151. See PHI654. When truncating strings like package names in the form "X: YYYYYYYYYYYY", we currently render "X...".

A better rendering in this case is "X: YYYYYY...", where we chop the long word in the middle.

Tweak the logic a bit to handle this case better.

This will support improving the display behavior of long package names in the table of contents view.

Test Plan: Added/modified unit tests, got a clean result.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19477