.arcconfig

rebase: check whether the rebasestate exists or not a bit early

Converted the else part into `if True` because that part will soon be under a
except part.

Differential Revision: https://phab.mercurial-scm.org/D3876

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