unbundle: free temporary objects after use

This reduces peak RSS for larger unbundle operations by ~30 Bytes per
changeset on AMD64. This can't be a direct delete for Python 2.7, so
reset the object instead and leave a comment.

The efilesset object can't be deleted as it is referenced by the local
onchangelog function and Python 2.7 rejects a delete on the existance of
a nested scope.

Differential Revision: https://phab.mercurial-scm.org/D9153

mercurial/changegroup.py

@@ -333,6 +333,10 b' class cg1unpacker(object):'
             clend = len(cl)
             changesets = clend - clstart
             progress.complete()
+            del deltas
+            # TODO Python 2.7 removal
+            # del efilesset
+            efilesset = None
             self.callback = None
 
             # pull off the manifest group