diff --stat: report mode-only changes for binary files like text files

diff --stat: report mode-only changes for binary files like text files

Mode-only changes to binary files without content change were reported as
if they were rewritten, but text files in the same situation were reported
as "unchanged". Let's treat binary files like text files here, and simply
say that they are unchanged.

Output of --shortstat is modified in the same way.

Reported-by: Martin Mareš <mj@ucw.cz>
Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>