.arcconfig

extdiff: register the configuration generated commands with a help category

Otherwise, 'extdiff' shows up under file management and the rest of the commands
are at the bottom under 'Uncategorized'.

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