Before this change, `jj new` would check out the new commit only if it
was created on top of the current commit. I never liked that
special-casing, and after thinking more about how the open/closed
should work (see discussion #321), I think we want `jj new` to behave
similar to how `git/hg checkout` works, so it can effectively replace
the current `jj checkout` command for the use case of starting new
work on top of an existing commit.
It can be confusing that some commits (typically the working copy)
don't have a description. Let's show a placeholder text in such cases.
I chose the format to match the "(no email configured)" message we
already have.
It's unusual for the current commit to have descendants, but it can
happen. In particular, it can easily happen when you run `jj new`. You
probably don't want to abandon it in those cases.
