tests/test-gendoc-ru.t

nodemap: add a (python) index class for persistent nodemap testing

Using the persistent nodemap require a compeling performance boost and an
existing implementation. The benefit of the persistent nodemap for pure python
code is unclear and we don't have a C implementation for it. Yet we would like
to actually start testing it in more details and define an API for using that
persistent nodemap.

We introduce a new `devel` config option to use an index class dedicated to
Nodemap Testing. This feature is "pure" only because having using a pure-python
index with the `cext` policy proved more difficult than I would like.

There is nothing going on in that class for now, but the coming changeset will
change that.

Differential Revision: https://phab.mercurial-scm.org/D7840

#require docutils gettext

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