graph: detect git branches and colourise them properly without rainbow effect (Issue #188) This is based on research and patches by Andrew Shadura. When using Git, only commits pointed to by branch references have their branches set. In general, there's no way in Git to find out which branch a commit belongs to, so we can try to deduce that from the merge topology. In other words, if we know the branch name, we know it's a different colour than any different branch. If we don't (it is None), we guesstimate the first parent is probably on the same branch. The relevant part of the code before 2da0dc09 used a similar, yet simpler, algorithm.