triplet_sort: handle new-style compose identifiers
    
    triplet_sort is no longer reliable on its own, as we are going
    to intersperse nightlies with productions, so its assumption
    that nightlies are always older than productions is invalid.
    However, it can be used as a tie-breaker when two events have
    the same creation_date, so we still want it to work as well as
    possible.