Once we add support for immutable commits, `jj duplicate` should be
allowed to create duplicate of them. The reason it can't duplicate the
root commit is that it would mean there would be multiple root
commits, which would break the invariant that the single root commit
is the only root commit (and the backends refuse to write a commit
without parents). So let's have `jj duplicate` check specifically that
the user doesn't try to duplicate the root commit instead.
They are shown next to the change and commit id, since they are other names the
commit can be referred by.
The description is separated from the branches by a ` | ` when there are
branches, so that one can tell the branches from the description without color.
The result looks like this: 
