Make project's Git repository a window global

This allows it to operate independently of the
project and mirror the availability of the root view
and project.

src/app/buffer.coffee

@@ -419,12 +419,10 @@ class Buffer
         return match[0][0] != '\t'
     undefined
 
-  getRepo: -> @project?.repo
-
   checkoutHead: ->
     path = @getPath()
     return unless path
-    if @getRepo()?.checkoutHead(path)
+    if git?.checkoutHead(path)
       @trigger 'git-status-changed'
 
   scheduleStoppedChangingEvent: ->