tests: Add `--rhg` and `--with-rhg=<path>` options for `run-tests.py`
They are mostly equivalent to the corresponding `chg` options.
For now, many tests are still failing in this configuration.
It is *not* run on CI.
Differential Revision: https://phab.mercurial-scm.org/D10095
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors