patch: add close() to abstractbackend

patchbackend() seems to call it on an arbitrary backend, so it seems
to be part of the API. Since all subclasses do something in their
close() methods, I decided to let this one raise an exception rather
than just pass.

mercurial/patch.py

@@ -448,6 +448,9 b' class abstractbackend(object):'
     def exists(self, fname):
         raise NotImplementedError
 
+    def close(self):
+        raise NotImplementedError
+
 class fsbackend(abstractbackend):
     def __init__(self, ui, basedir):
         super(fsbackend, self).__init__(ui)