rhg: add limited support for the `config` sub-command
Only with one argument and no flag. This is mostly for testing.
Differential Revision: https://phab.mercurial-scm.org/D9972
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}