Consolidate Git status checking

Now the status bar and tree view both listen for
status change events and use the cached information
available from the git object to update their views.

src/app/window.coffee

@@ -69,9 +69,11 @@ window.shutdown = ->
     rootView: rootView.serialize()
   rootView.deactivate()
   project.destroy()
+  git?.destroy()
   $(window).off('focus blur before')
   window.rootView = null
   window.project = null
+  window.git = null
 
 window.installAtomCommand = (commandPath) ->
   return if fs.exists(commandPath)