.arcconfig

chgserver: add "setumask2" command which uses correct message frame

The first 4 bytes should be a length field, not a value. Spotted while
porting chg functions to the Rust one.

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