upgradepath: fix the algorithm when pushing to older -updates repo
    
    When pushing to older -updates repo, we have to unite -main and -updates
    repos of higher Fedora releases to perform upgradepath checking
    correctly.
    
    Also this patch takes into account the rare case when someone would want
    to push a build that is older the the current one in a repository.
    
    Fixes tickets #309 and #230.