Update documentation to remove incorrect GIT_DIFF_OPTS example.

Git no longer calls an external diff program to generate patches.
Remove the documentation which suggests that you can pass
arbitrary diff options via the GIT_DIFF_OPTS environment variable.

Signed-off-by: Sean Estabrooks <seanlkml@sympatico.ca>
Signed-off-by: Junio C Hamano <junkio@cox.net>