`wc_commit` seems clearer than `checkout` and not too much longer. I
considered `working_copy` but it was less clear (could be the path to
the working copy, or an instance of `WorkingCopy`). I also considered
`working_copy_commit`, but that seems a bit too long.
In 8ae9540f2c, I made `jj move/squash/unsquash` not abandon the
working copy if it became empty because that would lose any
description associated with it. It turned out that the new behavior
was also confusing because it made it unclear if the working-copy
commit was actually abandoned. Let's roll back that change and instead
ask the user for a combined description when both the source and
destination commits have non-empty descriptions. Not discarding a
non-empty description seems like a good improvement regardless of the
behavior related to working-copy commits. It's also how `hg fold`
behaves (though hg doesn't allow the description to be empty).
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.
