Patch Set 1:

This doesn't yet work, it's a miracle to me why PlotWalk.getTags returns null for commits having refs, I can see in the debugger that ReverseRefMap contains entries (HEAD, refs/heads/master, refs/remotes/origin/master) for the id master points at but nevertheless reverseRefMap.get(commitId) returns null when passing in this same id.
:-(

Probably I am a bit too tired to spot the problem now ...