tests: check --[short]stat output after chmod

tests: check --[short]stat output after chmod

Add a test to check 'diff --stat' output with a text file after chmod,
and the same for a binary file. This demonstrates that text and binary
files are treated differently, which can be misleading.

While at it, add tests to check --shortstat output, too.

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>