prio_queue_reverse: don't swap elements with themselves

prio_queue_reverse: don't swap elements with themselves

Our array-reverse algorithm does the usual "walk from both
ends, swapping elements". We can quit when the two indices
are equal, since:

  1. Swapping an element with itself is a noop.

  2. If i and j are equal, then in the next iteration i is
     guaranteed to be bigge than j, and we will exit the
     loop.

So exiting the loop on equality is slightly more efficient.
And more importantly, the new SWAP() macro does not expect
to handle noop swaps; it will call memcpy() with the same src
and dst pointers in this case. It's unclear whether that
causes a problem on any platforms by violating the
"overlapping memory" constraint of memcpy, but it does cause
valgrind to complain.

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