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 useful for testing to be able to specify some operation that's
not the latest one.
I didn't update the changelog because this feature is mostly for
testing.
This involved copying `UnresolvedHeadRepo::resolve()` into the CLI
crate (and modifying it a bit to print number of rebased commit),
which is unfortunate.
