.arcconfig

phabricator: use .arcconfig for `phabricator.url` if not set locally

This setting is also per repo; see the previous commit for details.

The existing `conduit_uri` setting is the previous name of `phabricator.uri`[1]
and while it could easily be queried before the latter for compatibility, the
config in this repo has '/api' appended. That's already done in `callconduit()`,
which would clearly end up giving the wrong result. It looks like the path of
the URL is now ignored in user configs[2], so add the modern setting without it
to this repo's .arcconfig.

Sadly, we still need to have contributors configure `auth.hg.phabtoken` (and
therefore `auth.hg.prefix` to link it to `phabricator.url`) in order to submit
patches, but at least now it's localized to a single section.

[1] https://secure.phabricator.com/book/phabricator/article/arcanist_new_project/
[2] https://github.com/phacility/arcanist/blob/cc850163f30c4697e925df0d6212469679600a2c/scripts/arcanist.php#L271

Differential Revision: https://phab.mercurial-scm.org/D7935

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}