tests/test-gendoc-zh_TW.t

phases: apply similar early filtering to advanceboundary

advanceboundary is called the push's unbundle (but not the other unbundle) so
advanceboundary did not show up the profile I looked at so far.

We start with simple pre-filtering to avoid doing any work if we don't needs
too.

#require docutils gettext

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