Add git-diff orderFile

It's helpful to have a common order of file types in patches.
Applying git-diff's orderFile feature allows us to do that. Just
do 'git config diff.orderFile scripts/git.difforder' to enable it.

Acked-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>