lookup: don't use "00changelog.i@None" when lookup of prefix fails

We were shadowing the "node" variable, so we always passed None to the
LookupError instead of the node we meant to pass.

(This showed up in py3 tests since py3 doesn't like to format None
using "%s".)

Differential Revision: https://phab.mercurial-scm.org/D6661

mercurial/revlog.py

@@ -1355,13 +1355,13 b' class revlog(object):'
         """Find the shortest unambiguous prefix that matches node."""
         def isvalid(prefix):
             try:
-                node = self._partialmatch(prefix)
+                matchednode = self._partialmatch(prefix)
             except error.AmbiguousPrefixLookupError:
                 return False
             except error.WdirUnsupported:
                 # single 'ff...' match
                 return True
-            if node is None:
+            if matchednode is None:
                 raise error.LookupError(node, self.indexfile, _('no node'))
             return True