phabricator: include the suggested arc config in the repo

Test Plan:
I uploaded this revision with `arc diff`

Differential Revision: https://phab.mercurial-scm.org/D171

.arcconfig

@@ -0,0 +1,6 b''
+{
+    "conduit_uri": "https://phab.mercurial-scm.org/api",
+    "repository.callsign": "HG",
+    "arc.land.onto.default": "@",
+    "base": "hg:.^"
+}

tests/test-check-code.t

@@ -36,6 +36,7 b' New errors are not allowed. Warnings are'
 Prevent adding new files in the root directory accidentally.
 
   $ testrepohg files 'glob:*'
+  .arcconfig
   .editorconfig
   .hgignore
   .hgsigs