qsort(): ptrdiff_t may be larger than int

This is a companion patch to e23eff8be92a2a2cb66b53deef020063cff285ed
commit.  The same logic, the same rationale that a comparison
function that returns an int should not just compute a ptrdiff_t
and return it.

Signed-off-by: Junio C Hamano <junkio@cox.net>