.arcconfig

perf: add a --stats argument to perfhelper-mergecopies

The arguments will display some statistics about the distribution of the value
we measure.

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