diff,difftool: Don't use the {0,2} notation in usage strings

This was the only occurence of that usage, and square brackets are
sufficient and already well-established for that purpose.

Signed-off-by: Štěpán Němec <stepnem@gmail.com>
Acked-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>