The --quiet flag is not meant to be passed on to the diff, as the user
always wants the patches to be produced so catch it and pass it to
reopen_stdout which decides whether to print the filename or not.
Noticed by Paul Gortmaker
Signed-off-by: Carlos Martín Nieto <cmn@elego.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>