tests/test-gendoc-zh_TW.t

undo-files: also remove the undo.backupfiles

The undo.backupfiles is dealt is directly managed by the transaction instead of
going through the `localrepo.undofiles`. We start doing minimal management for
it before using `cleanup_undo_files` on more situation. Proper handling of it
is an intermediate goal of this series.

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_TW
  checking for parse errors