hg: improve implementations of `successors` and `precursors` properties of Mercurial changesets * On Mercurial versions supporting it, make the properties return the closest changesets in the obsolescence chain that are in the repository. * On older Mercurial versions, fall back to the previous implementations.

[tox]
minversion = 1.8
envlist = py{26,27}-pytest

[testenv]
setenv =
    PYTHONHASHSEED = 0
deps =
    -r{toxinidir}/dev_requirements.txt
    py26-pytest: unittest2
    python-ldap
    python-pam
commands =
    pytest: py.test {posargs}