Use fixed-size integers for the on-disk pack structure.

Use fixed-size integers for the on-disk pack structure.

Plain integer types without a fixed size can vary between platforms.  Even
though all common platforms use 32-bit ints, there is no guarantee that
this won't change at some point.  Furthermore, specifying an integer type
with explicit size makes the definition of structures more obvious.

Signed-off-by: Simon 'corecode' Schubert <corecode@fs.ei.tum.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>