[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