changelog: remove copy of revlog.nodemap()
It's been there since 2012, "clfilter: introduce `filteredrevs` attribute
on changelog." I don't think we can apply changelog filtering to nodemap at
this level, so this patch removes the nodemap stub completely.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.