mass-tag.py: Use rpm vercmp for determining what to tag over
    
    This should fix the issue we had with tagging back builds in the latest
    mass rebuild where the script tagged over a number of packages that had
    actually gotten rebuilt in rawhide in the mean time.
    
    Previously, the logic was to compare task start times, but the way it
    found the "newer" task failed to take into account that not all builds
    are done using XX-candidate/rawhide targets and failed to find builds
    done in self-service side tags.
    
    This commit simplifies the logic in the script and makes it just use rpm
    vercmp to compare the latest builds between the source and target tags
    to determine what to tag over, hopefully making it all less error prone.
    
    Signed-off-by: Kalev Lember <klember@redhat.com>