changegroup: use the local variable instead of reaching through self

Gratuitous cleanup while I was here.

Differential Revision: https://phab.mercurial-scm.org/D10023

mercurial/changegroup.py

@@ -1009,7 +1009,7 b' class cgpacker(object):'
         mfdicts = None
         if self._ellipses and self._isshallow:
             mfdicts = [
-                (self._repo.manifestlog[n].read(), lr)
+                (repo.manifestlog[n].read(), lr)
                 for (n, lr) in pycompat.iteritems(manifests)
             ]