index-pack usage of mmap() is unacceptably slower on many OSes other than Linux

index-pack usage of mmap() is unacceptably slower on many OSes other than Linux

It was reported by Randal L. Schwartz <merlyn@stonehenge.com> that
indexing the Linux repository ~150MB pack takes about an hour on OS x
while it's a minute on Linux.  It seems that the OS X mmap()
implementation is more than 2 orders of magnitude slower than the Linux
one.

Linus proposed a patch replacing mmap() with pread() bringing index-pack
performance on OS X in line with the Linux one.  The performances on
Linux also improved by a small margin.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>