.arcconfig

template: add minimal obsfate template function

The goal of this series is to have templates capable of displaying the
evolution of a changeset in a clean and human-readable way.

Add the succsandmarkers template return successors and markers so it can be
used separately like this:

> {succsandmarkers % "{get(succsandmarkers, "markers")|json};"}

The following patches will add template functions that takes successors and
markers as inputs and compute various obsfate fields from them.

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