rebase: change and standarize template for rebase's one-line summary 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? Differential Revision: https://phab.mercurial-scm.org/D9252