context: use rev for changelog lookup

Faster when we're doing numeric scanning

mercurial/context.py

@@ -129,7 +129,7 b' class changectx(object):'
 
     @propertycache
     def _changeset(self):
-        return self._repo.changelog.read(self.node())
+        return self._repo.changelog.read(self.rev())
 
     @propertycache
     def _manifest(self):