Break long words in differential two-up view

Summary: This should prevent long lines from making the code width different between files, which can be annoying. (And of course, it stops long lines from making a giant scrollbar too.)

Test Plan:
Loaded this diff in Chrome, Firefox, IE9, and IE8:

(That's a screenshot from Chrome, but it looks about the same in the other browsers.)

Reviewers: chad, Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin, chad

Maniphest Tasks: T2004

Differential Revision: https://secure.phabricator.com/D8686