ref-filter: use QSORT_S in ref_array_sort()

ref-filter: use QSORT_S in ref_array_sort()

Pass the array of sort keys to compare_refs() via the context parameter
of qsort_s() instead of using a global variable; that's cleaner and
simpler.  If ref_array_sort() is to be called from multiple parallel
threads then care still needs to be taken that the global variable
used_atom is not modified concurrently.

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