This is a naive implementation, which cannot deal with multiple children
or parents stemming from merges.
Note: I gave each command separate a separate argument struct
for extensibility.
Fixes#878
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.
