.arcconfig

requires: re-use vfs.tryread for simplicity

Avoids calling `set` twice or having to re-raise an exception and implements the routine with a single return expression.

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