Only generate compose changelogs once per day
    
    This will save one compose per day as the comparison base and then will
    only publish the results of the comparison on a new calendar day.
    
    The comparison logs will still be generated and available in the compose
    tree, but they won't be mailed to the lists.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>