phases: remove useless lookup of repo[rev].rev() in _retractboundary
changectx object is no longer needed thanks to
a54ee130210a.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}