Analyse gitlab-org/gitlab keep-around refs for pruning

Because git repositories consist of chains of commits, we know that if a commit is kept-around then all its ancestor commits are also kept.

In theory this could mean that if a keep-around ref is a decedent of another keep-around ref then it can be removed without risking any objects being garbage collected.

We should analyse some large repos such as gitlab-org/gitlab on .com to see how many keep-around refs could be pruned based on this method.