Get rid of the dependency to GNU diff in the tests

Now that "git diff" handles stdin and relative paths outside the
working tree correctly, we can convert all instances of "diff -u"
to "git diff".

This commit is really the result of

$ perl -pi.bak -e 's/diff -u/git diff/' $(git grep -l "diff -u" t/)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

(cherry picked from commit c699a40d68215c7e44a5b26117a35c8a56fbd387)

t/t4006-diff-mode.sh

@@ -38,7 +38,7 @@ echo ":100644 100755 X X M	rezrov" >expected
 
 test_expect_success \
     'verify' \
-    'diff -u expected check'
+    'git diff expected check'
 
 test_done