narrow: get the narrow patterns from the repository object instead of disk
Relying on disk data make the transactionally of this change complicated, so
let us start reading data from other API instead.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}