Through various refactorings over time, the extract(timezone_minute
from time with time zone) and extract(timezone_minute from timestamp
with time zone) implementations ended up with two different but
equally nonsensical formulas by using SECS_PER_MINUTE and
MINS_PER_HOUR interchangeably.  Since those two are of course both the
same number, the formulas do work, but for readability, fix them to be
semantically correct.