obsolete: track markers in _succs
We now also store markers in _succs. It will be useful for the obsfate template that
will use them to display more meaningful information like the list of users
that have evolved a changeset into its successors.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.