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

.arcconfig

@@ -1,5 +1,6 b''
 {
     "conduit_uri": "https://phab.mercurial-scm.org/api",
+    "phabricator.uri": "https://phab.mercurial-scm.org/",
     "repository.callsign": "HG",
     "arc.land.onto.default": "@",
     "base": "hg:.^"

hgext/phabricator.py

@@ -178,11 +178,16 b' def _loadhgrc(orig, ui, wdirvfs, hgvfs, '
     except IOError:
         pass
 
+    cfg = util.sortdict()
+
     if b"repository.callsign" in arcconfig:
-        ui.applyconfig(
-            {(b"phabricator", b"callsign"): arcconfig[b"repository.callsign"]},
-            source=wdirvfs.join(b".arcconfig"),
-        )
+        cfg[(b"phabricator", b"callsign")] = arcconfig[b"repository.callsign"]
+
+    if b"phabricator.uri" in arcconfig:
+        cfg[(b"phabricator", b"url")] = arcconfig[b"phabricator.uri"]
+
+    if cfg:
+        ui.applyconfig(cfg, source=wdirvfs.join(b".arcconfig"))
 
     return orig(ui, wdirvfs, hgvfs, requirements) or result  # Load .hg/hgrc
 

tests/test-phabricator.t

@@ -228,8 +228,8 b' The .arcconfig content overrides global '
   $ mv .hg/hgrc .hg/hgrc.bak
   $ hg config phabricator --debug
   read config from: */.hgrc (glob)
-  */.hgrc:*: phabricator.url=global (glob)
   $TESTTMP/repo/.arcconfig: phabricator.callsign=HG
+  $TESTTMP/repo/.arcconfig: phabricator.url=https://phab.mercurial-scm.org/
 
 But it doesn't override local config
   $ cat >> .hg/hgrc << EOF