tests/test-gendoc-zh_TW.t

docs: prefer `hg diff --from/--to` over `-r`

This patch includes updating away from the broken `hg diff -r
'date(...)'` (see not in previous patch).

Differential Revision: https://phab.mercurial-scm.org/D9555

#require docutils gettext

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