Patrick Steinhardt [off-list ref] writes:

We can:

    $ git update-ref $NEW_OID $NULL_OID
    $ git refs update $NEW_OID $NULL_OID

This will verify that the reference doesn't exist before actually
writing it. Will add a test.

I think refname is missing from the command line, but the above is
good.  I forgot we had update-ref already doing that ;-)

Thanks.