.arcconfig

strip: use in-place revset formatspec instead of %-formatting ourselves

Caught by Yuya during review of D1884.

Differential Revision: https://phab.mercurial-scm.org/D1905

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