Make loose object file reading more careful

Make loose object file reading more careful

We used to do 'stat()+open()+mmap()+close()' to read the loose object
file data, which does work fine, but has a couple of problems:

 - it unnecessarily walks the filename twice (at 'stat()' time and then
   again to open it)

 - NFS generally has open-close consistency guarantees, which means that
   the initial 'stat()' was technically done outside of the normal
   consistency rules.

So change it to do 'open()+fstat()+mmap()+close()' instead, which avoids
both these issues.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>