With the idea that less severe placeholders (like description) could
(and should) explicitly "opt out".
(Both email and name placeholders will be red with this change.)
This patch also extracts format_detailed_signature() function to deduplicate
the "show" template bits.
The added placeholder templates aren't labeled as "empty". If needed, I think
the whole template can be labeled as "empty" (or "empty_commit") just like
"working_copy".
Closes#2112
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.
