compat: Add simplified merge sort implementation from glibc

compat: Add simplified merge sort implementation from glibc

qsort in Windows 2000 (and various other C libraries) is a Quicksort
with the usual O(n^2) worst case.  Unfortunately, sorting Git trees
seems to get very close to that worst case quite often:

    $ /git/gitbad runstatus
    # On branch master
    qsort, nmemb = 30842
    done, 237838087 comparisons.

This patch adds a simplified version of the merge sort that is glibc's
qsort(3).  As a merge sort, this needs a temporary array equal in size
to the array that is to be sorted, but has a worst-case performance of
O(n log n).

The complexity that was removed is:

* Doing direct stores for word-size and -aligned data.
* Falling back to quicksort if the allocation required to perform the
  merge sort would likely push the machine into swap.

Even with these simplifications, this seems to outperform the Windows
qsort(3) implementation, even in Windows XP (where it is "fixed" and
doesn't trigger O(n^2) complexity on trees).

[jes: moved into compat/qsort.c, as per Johannes Sixt's suggestion]
[bcd: removed gcc-ism, thanks to Edgar Toernig.  renamed make variable
      per Junio's comment.]

Signed-off-by: Brian Downing <bdowning@lavos.net>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>