Speed dictresize by collapsing its two passes into one; the reason given

Speed dictresize by collapsing its two passes into one; the reason given
in the comments for using two passes was bogus, as the only object that
can get decref'ed due to the copy is the dummy key, and decref'ing dummy
can't have side effects (for one thing, dummy is immortal!  for another,
it's a string object, not a potentially dangerous user-defined object).