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