cfbot vs. changes in the set of CI tasks

I was confused about how come the new patches I'd just posted in
the 3848 CF item (write/read support for raw parse trees) are
showing a mix of passes and fails in the cfbot.  I eventually
realized that the fail results are all old and stale, because
(for example) there's no longer a "FreeBSD - 13" task to run,
just "FreeBSD - 13 - Meson".  This seems tremendously confusing.
Can we get the cfbot to not display no-longer-applicable results?

As a quick-fix hack it might do to just flush all the pre-meson-commit
results, but I doubt this'll be the last time we make such changes.
I think an actual filter comparing the result's time to the time of the
allegedly-being-tested patch would be prudent.

            regards, tom lane