Uncollapse side panel after enabling file tree

Summary:
If user enables file tree but had toggled the side panel with 'F' before, it didn't show up.
It is confusing.

Test Plan: With hidden panel, enabled filetree.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5160