drawdag: correctly pass repo to super constructor

This was caught by someone who was looking for the

  super(cls, self).__init__(self, ...)

pattern in all code in our Google-internal repo.

Differential Revision: https://phab.mercurial-scm.org/D4464

tests/drawdag.py

@@ -288,8 +288,7 b' class simplecommitctx(context.committabl'
             'date': b'0 0',
             'extra': {b'branch': b'default'},
         }
-        super(simplecommitctx, self).__init__(self, name, **opts)
-        self._repo = repo
+        super(simplecommitctx, self).__init__(repo, name, **opts)
         self._added = added
         self._parents = parentctxs
         while len(self._parents) < 2: