File tree improvements
Created by: attfarhan
Addresses outstanding comments in https://github.com/sourcegraph/sourcegraph/pull/9842.
Also fixes a small unintended bug in #9842. When the go-to-def or find refs button is clicked, the ?subtree
param would be included in the URL, so when the references or definition panel appears, the file tree updates to only show the current directory, which is not what we want. So I check to make sure the tab=
parameter is not present before updating the tree. cc @uwedeportivo I would like to get this cherry-picked into the release.