Select current branch in status images popup.

This is an attempt to implement #53 feature request: select current build's branch in the status images popup by default. It doesn't reset selected branch each time the popup is open, so until you switch to a different build, popup will 'remember' selected branch.

I tried to use view's `'branches'` property instead of `'repo.branches'`, but for some reason the resulting behavior was pretty weird. Probably because of this `if @get('active')` condition in the `'branches'` property definition.