Test diff --dirstat functionality

This is only a very rudimentary test, but it was untested before.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013-diff-various.sh

@@ -263,6 +263,7 @@ diff --name-status dir2 dir
 diff --no-index --name-status dir2 dir
 diff --no-index --name-status -- dir2 dir
 diff master master^ side
+diff --dirstat master~1 master~2
 EOF
 
 test_done