createrepo: Only consider successful compose for deltas
    
    If the compose failed, it may not have repos to compute deltas against,
    and even if it has them, they were never shipped so no one will have the
    older version of the package. We should instead go deeper in history and
    pick a successful compose.
    
    Relates: https://pagure.io/pungi/issue/715
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>