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
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}