Slightly improve prose diffs for text ending in newlines or punctuation

Summary:
Ref T3353. Currently, if text ends in newlines (or some other types of punctuation) we generate an extra piece at the end which can muck up the algorithm.

Do a slightly better job with this.

Test Plan: Viewed a diff between "a\nb\nc\n" and "a\nb\nc\nd\n", got a cleaner diff out of it.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3353

Differential Revision: https://secure.phabricator.com/D16061