nodemap: rename a variable to prevent shawoding "_"

This will make pyflakes happy with the next changeset.

Differential Revision: https://phab.mercurial-scm.org/D8412

mercurial/revlogutils/nodemap.py

@@ -519,7 +519,7 b' def _walk_trie(block):'
 
     Children blocks are always yield before their parent block.
     """
-    for (_, item) in sorted(block.items()):
+    for (__, item) in sorted(block.items()):
         if isinstance(item, dict):
             for sub_block in _walk_trie(item):
                 yield sub_block