Add GitRepository.prototype.isDestroyed

src/git-repository.coffee

@@ -130,6 +130,10 @@ class GitRepository
       @async.destroy()
       @async = null
 
+  # Public: Returns a {Boolean} indicating if this repository has been destroyed.
+  isDestroyed: ->
+    not @repo?
+
   # Public: Invoke the given callback when this GitRepository's destroy() method
   # is invoked.
   #