revlog.revision(): don't use nullrev as the default value for the cache

I is probably a bug if the deltachain computation think there was a cache hit
at nullrev. Use None instead, this will never trigger a cache hit.

mercurial/revlog.py

@@ -1056,7 +1056,7 b' class revlog(object):'
 
     def revision(self, node):
         """return an uncompressed revision of a given node"""
-        cachedrev = nullrev
+        cachedrev = None
         if node == nullid:
             return ""
         if self._cache: