[PATCH] Use diff-tree -p -r instead of "git diff" in git-export.

Now diff-tree can produce patch itself, there is no reason to
depend on Cogito to show diff in the git-export output anymore.

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