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.
(binary:application/octet-stream)