Differential - add an undo element when you collapse a file

Summary: Fixes T2258.

Test Plan: collapsed and expanded file via the dropdown - good stuff. got the "undo" element into the mix - also good stuff.

Reviewers: epriestley

Reviewed By: epriestley

CC: chad, Korvin, aran

Maniphest Tasks: T2258

Differential Revision: https://secure.phabricator.com/D6742