.arcconfig

context: split `introrev` logic in a sub function

We want to add a mechanism to stop iteration early associated to intro rev early
in some case. However, it does not make sense to expose it in the public
`filectx` API. So we split the code into an internal method instead.

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