Step 1 ruled what a rollup entry means and never said which entry to
read. A job in a cancel-in-progress group displaces itself, so a head
routinely carries a CANCELLED node beside the SUCCESS that replaced it,
and read by class alone that head is not green while checks_state calls
it SUCCESS.
State the collapse ahead of the classes it feeds: newest entry by start
time, and a CANCELLED entry is not the check's word while a non-cancelled
sibling stands at the same head. All-cancelled and pending are untouched.
Refs #276
