Better JS for Differential File Collapsing Undo

Summary: take epriestley's feedback 'cuz its good

Test Plan: collapse, expand, use undo like a rockstar. observe proper behavior

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T2258

Differential Revision: https://secure.phabricator.com/D6748