pass constants as first argument to st_mult()

pass constants as first argument to st_mult()

The result of st_mult() is the same no matter the order of its
arguments.  It invokes the macro unsigned_mult_overflows(), which
divides the second parameter by the first one.  Pass constants
first to allow that division to be done already at compile time.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>