Make git diff-generation use a simpler spawn-like interface

Make git diff-generation use a simpler spawn-like interface

Instead of depending of fork() and execve() and doing things in between
the two, make the git diff functions do everything up front, and then do
a single "spawn_prog()" invocation to run the actual external diff
program (if any is even needed).

This actually ends up simplifying the code, and should make it much
easier to make it efficient under broken operating systems (read: Windows).

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>