.arcconfig

rust-chg: add parser for request messages sent to "S" channel

The data structure is documented at chgserver.py:channeledsystem().

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