ostree: Send correct ref in the message
    
    If configuration requests a different ref than what treefile has, a
    wrong message can be sent.
    
    If the treefile is in JSON, the local copy will be updated in-place and
    it works. However with YAML the updated version is still written as JSON
    and thus we are still sending the original value.
    
    Fixes: https://pagure.io/pungi/issue/1092
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>