diff -r 2d4cad94d08a -r ab595920de0e mercurial/revlogutils/__init__.py
--- a/mercurial/revlogutils/__init__.py	Sun Dec 29 21:10:04 2019 -0500
+++ b/mercurial/revlogutils/__init__.py	Tue Jan 07 12:26:37 2020 +0100
@@ -6,9 +6,3 @@
 # GNU General Public License version 2 or any later version.
 
 from __future__ import absolute_import
-from .. import error
-
-
-class NodeMap(dict):
-    def __missing__(self, x):
-        raise error.RevlogError(b'unknown node: %s' % x)