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:.^"
}