This allow the phabricator extension to read arc config files to auto-configure
url, token and callsign.
We use it as a fallback when phabricator.url or phabricator.callsign aren't
This allow to configure conduit_uri and repository.callsign in a tracked
.arcconfig json file in the root of the repository, so users having lot of
small repositories in phabricator doesn't need to configure .hg/hgrc after a