Fix scrollbar in filetree sidenav

Summary: Fixes T11457, I guess. It feels wierd when you drag it a bit, and Im not positive if it's the calc css or javascript.

Test Plan: Enable filetree, shorten browser, see scrolling appear if files too long.

Reviewers: avivey, epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11457

Differential Revision: https://secure.phabricator.com/D16393