.arcconfig

rebase: make "successors" a set in _computeobsoletenotrebased()

There's no apparent reason for this variable to be a list and this
avoids converting it to a set when needed.

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