The low-level diff code will happily produce totally bogus diff output
with a broken repository via format-patch and friends by treating missing
objects as empty files. Let's prevent that from happening any longer.
Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de> Signed-off-by: Nicolas Pitre <nico@fluxnic.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>