Ability to compare against existing run #198
Description
On backports branches, it is common to compare against a release branch that's unlikely to change. Thus it may be beneficial to support comparing against the reference of a previous PkgEval run, instead of having to re-build and re-test all those packages. This probably requires
- storing more data in the repository again
- the ability to pin the registry to avoid package updates
I don't think we should do this automatically; it's probably too automatic to sometimes use an outdated registry just for the sake of reusing a previous evaluation job (and often we explicitly want to re-test in order to flush out spurious errors). I guess something like runtests(vs = "%previous") could be a good syntax for this.
cc @KristofferC