ci: add address and undefined sanitizer tasks

ci: add address and undefined sanitizer tasks

The current code is clean with these two sanitizers, and we would
like to keep it that way by running the checks for any new code.

The signal of "passed with asan, but not ubsan" (or vice versa) is
not that useful in practice, so it is tempting to run both santizers
in a single task, but it seems to take forever, so tentatively let's
try having two separate ones.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>