revsets: define a none() revset
This can be useful for default values in corp-wide configuration
(e.g. for commits to hide, which defaults to none()), and perhaps in
scripts. I named it none() rather than empty() to match all().
Differential Revision: https://phab.mercurial-scm.org/D3713
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)