rhg: Pass a &Config to Ui::new
When a Ui object is needed to print errors about configuration-loading errors,
an empty (default) configuration is used.
Differential Revision: https://phab.mercurial-scm.org/D12164
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}