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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true