Give prose diffs some basic test coverage

Summary: Ref T7643. This strategy looks like it's a reasonable way to get these diffs to "pretty good, most of the time" levels. Since I'm not plannign to throw it away, start adding some test coverage.

Test Plan: Ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7643

Differential Revision: https://secure.phabricator.com/D16067