vcs.mercurial: Shell out to hg instead of using the mercurial package

vcs.mercurial: Shell out to hg instead of using the mercurial package

Mercurial doesn't have Python 3 support, and has no plans for adding
it in the near future [1].  It's a pretty painless conversion to shell
out instead (as we already do for Git), so go that route instead.

[1]: http://mercurial.selenic.com/wiki/SupportedPythonVersions#Python_3.x_support