tests: do not use implicit "git diff --no-index"

As a general principle, we should not use "git diff" to validate the
results of what git command that is being tested has done.  We would not
know if we are testing the command in question, or locating a bug in the
cute hack of "git diff --no-index".

Rather use test_cmp for that purpose.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4118-apply-empty-context.sh

@@ -38,7 +38,7 @@ test_expect_success 'apply --numstat' '
 		echo "0	1	file1" &&
 		echo "0	1	file2"
 	} >expect &&
-	git diff expect actual
+	test_cmp expect actual
 
 '
 
@@ -48,8 +48,8 @@ test_expect_success 'apply --apply' '
 	cat file2.orig >file2 &&
 	git update-index file1 file2 &&
 	git apply --index diff.output &&
-	git diff file1.mods file1 &&
-	git diff file2.mods file2
+	test_cmp file1.mods file1 &&
+	test_cmp file2.mods file2
 '
 
 test_done