censor: make rhg fall back to python when encountering a censored node
This is to make it support censor.policy=ignore without having
to duplicate that logic.
Also, change the censor test in such a way that it uses rhg now,
because extensions are disabled except when we call [hg censor].
Differential Revision: https://phab.mercurial-scm.org/D12607
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors