I think this is less surprising than falling back to the default length.
i64-to-usize conversion can also overflow on 32 bit environment, but I'm not
bothered to handle overflow scenario.
This allows negative numbers, which also means functions which took numbers can now take negative numbers
Luckily, they all already handled this exactly as expected.
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.
