No diff -b/-w output for all-whitespace changes

Change git-diff's whitespace-ignoring modes to generate
output only if a non-empty patch results, which git-apply
rejects.

Update the tests to look for the new behavior.

Signed-off-by: Greg Bacon <gbacon@dbresearch.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4015-diff-whitespace.sh

@@ -93,8 +93,6 @@ git diff > out
 test_expect_success 'another test, without options' 'test_cmp expect out'
 
 cat << EOF > expect
-diff --git a/x b/x
-index d99af23..8b32fb5 100644
 EOF
 git diff -w > out
 test_expect_success 'another test, with -w' 'test_cmp expect out'
@@ -386,6 +384,18 @@ test_expect_success 'checkdiff allows new blank lines' '
 	git diff --check
 '
 
+cat <<EOF >expect
+EOF
+test_expect_success 'whitespace-only changes not reported' '
+	git reset --hard &&
+	echo >x "hello world" &&
+	git add x &&
+	git commit -m "hello 1" &&
+	echo >x "hello  world" &&
+	git diff -b >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'combined diff with autocrlf conversion' '
 
 	git reset --hard &&