phases: rename variable used for owned dict of phasesets
The phaseroots variable is used for two different objects: borrowed set
and owned dict of sets. It's hard to track which object should have to be
decrefed on error return.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}