vfs: treat 'undo.' file the same as 'journal.' file

They are the same kind of file, they are protected by the store lock, but
directly lives inside the '.hg' directory.

No warnings were ever raised about them because `vfs.rename` is not audited.
Something we are trying to change.

mercurial/localrepo.py

@@ -1031,7 +1031,7 b' class localrepository(object):'
             if path.startswith('cache/'):
                 msg = 'accessing cache with vfs instead of cachevfs: "%s"'
                 repo.ui.develwarn(msg % path, stacklevel=2, config="cache-vfs")
-            if path.startswith('journal.'):
+            if path.startswith('journal.') or path.startswith('undo.'):
                 # journal is covered by 'lock'
                 if repo._currentlock(repo._lockref) is None:
                     repo.ui.develwarn('write with no lock: "%s"' % path,