tests/test-gendoc-zh_CN.t

unstable: use the `_mutablerevs` function when computing content divergent

This is a useful function to get the revision relevant to these computation,
lets make sure all code use it, so that we can improve that `_mutablerevs`
function in a later changeset.

#require docutils gettext

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