Bright green really pops on my screen, and I don't think there is a reason
for the root commit to be attention-grabbing.
This follows up on https://github.com/martinvonz/jj/pull/2084.
I've extracted the `builtin_log_root` template for users to customize the
default templates without fully overriding them, for example I would remove
the change_id/commit_id for myself - and we discussed in Discord that leaving
those makes sense for the user to be reminded/teached that the root commit has
a change id made from z's.
