Overflow differential content

Summary: Allow individual changesets to overflow-x: auto, Fixes T4164

Test Plan: view large diff, see some scrollbars, some without

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4164

Differential Revision: https://secure.phabricator.com/D7855