Patch Set 4: Looks good to me, but someone else must approve; IP review completed

I fixed some bugs for layouting complicated histories. Now I am quite happy with it. We have good tests, I tried it with linux and git-git histories and at least it doesn't show wrong topologies.
When looking at big, complicated repos (like linux) our history is using substantially more lanes than e.g. gitk. There are some tricks in gitk which don't have in yet - but at least the display is correct now.