better logging
resolve #39, handle tags pointing to tags!
Turns out a tag MAY point to another tag object. Then we must unpack
all the references until we get the final commit it - as we need to
group by on it (create the map[Id, List[Tag]]).

This has been detected by watching the repo
https://github.com/gufi/OpenLogViewer around
0.0.3-SNAPSHOT and OpenLogViewer-0.0.2;

Kudos to @fredcooke for helping locate this edge case