This removes the default template in rebase and switches to a
centrally defined template. I've simplified it a bit to avoid the
conditional parenthesis. I've also added labels so the different parts
can be easily colored. The template is somewhat similar to what we've
used internally at Google for a few years.
I'm happy to change the template if others have opinions. Should we
reuse the color.log. names as I have?
I left out branches and custom namespaces on purpose because I figured people like us (Google) who have custom namespaces can also have custom configs. However, I just realized that this makes everyone with the topic extension lose the topic they've had in rebase output for a long time (ever since someone was nice enough to add it in D741). I'll send a follow-up to include names from all non-branch namespaces. Sorry about the churn that that will incur.