.arcconfig

graft: add test for reading old graftstate files with new mechanism

This tests the reading of old graftstate file using the new logic. The tests
shows that if user is in middle of a graft and then updates their mercurial to
the version where we have new graftstate format, we can still read the old graft
state format files correctly.

Differential Revision: https://phab.mercurial-scm.org/D2597

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