If the source commit becomes empty as a result of
`move/squash/unsquash`, we abandon it. However, perhaps we shouldn't
do that if the source commit is a working-copy commit because
working-copy commits are often work-in-progress commits.
The background for this change is that @arxanas had just started a new
change and had set a description on it, and then decided to make some
changes in the working copy that should be in the parent
commit. Running `jj squash` then abandoned the working-copy commit,
resuling in the description getting lost.
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.
