stable-qsort: avoid using potentially unaligned access

stable-qsort: avoid using potentially unaligned access

Like in the previous patch for compat/qsort_s.c, remove the optimization
of using an on-stack buffer to avoid small allocations.  This ensures
maximum alignment for the array elements and simplifies the code a bit.

The performance impact for the current callers is unlikely to be
noticeable:

 * compat/mingw.c::make_environment_block() uses ALLOC_ARRAY and
   ALLOC_GROW several times already, so another allocation of up to 1KB
   should not matter much.

 * diffcore-rename.c::diffcore_rename_extended() is called once per diff
   or twice per merge, and those require allocations for each object and
   more already.

 * merge-ort.c::detect_and_process_renames() is called once per merge.
   It's responsible for the two per-merge diffcore_rename_extended()
   calls mentioned above as well, though.  So this is possibly the most
   impacted caller.  Per-object allocations are likely to dwarf the
   additional small allocations in git_stable_qsort(), though.

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