packaging: modify rc detection to work with X.Yrc instead of X.Y-rc
rc detection on CentOS failed without this change,
resulting in upgrades from 4.9rc to 4.9 not working
(4.9rc was considered more recent than 4.9).
Differential Revision: https://phab.mercurial-scm.org/D5876
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}