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