[Redesign] Fix filetree view in Differential
Summary: Ref T8099. We removed the background column, so this JS wasn't able to fire and make the filetree movable.
Test Plan: Set Filetree on, resize it. Scroll through a diff.
Reviewers: btrahan, epriestley
Reviewed By: epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T8099
Differential Revision: https://secure.phabricator.com/D13048