Repaint filetree more consistently for mobile/device views

Summary:
Ref T13189. See https://discourse.phabricator-community.org/t/diffusion-differential-mobile-layout-broken-when-enabling-file-tree/1751.

We currently call a nonexistent resetdrag() which does nothing. Some sequences of interactions can result in a blank left column in mobile/device widths.

Repaint the filetree away more consistently on device change.

Test Plan: Viewed a revision, toggled filetree off + on, resized to narrow width. Before: bad left margin, JS console error. After: proper repaint at device breakpoint, no JS console error.

Reviewers: amckinley

Maniphest Tasks: T13189

Differential Revision: https://secure.phabricator.com/D19611