Create some visual pictures / diagrams to show what git debrebase is doing

We use git debrebase commands like new-upstream and conclude and launder, but there's currently only a lot of text that describes what they do. For the visual thinkers, it's currently really hard to grasp what the tool is actually doing to the git history. This is especially the case since gdr is doing non-standard stuff like forced fake merges, and since the resulting history looks like a rainbow, or christmas tree by default.

So, for a few of our use cases, create some visuals (e.g. 'bolletje-pijltje' style) that show the 1-2-3 steps.

• Do a security update (only upstream changes)
• Do a Debian unstable update (to newer upstream commit, and with a few changes in debian/)
• Do an update to a major new Xen version that first goes to experimental and then later is merged back to sid.