rebase: add basic coloring output

This uses coloring style like we use in grep/forall already.

Change-Id: I317e2e47567a30c513083c48e7c7c40b091bb29a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/238555
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>