diff: correct interaction between --exit-code and -I<pattern>

Just like "git diff -w --exit-code" should exit with 0 when ignoring
whitespace differences results in no changes shown, if ignoring
certain changes with "git diff -I<pattern> --exit-code" result in an
empty patch, we should exit with 0.

The test suite did not cover the interaction between "--exit-code"
and "-w"; add one while adding a new test for "--exit-code" + "-I".

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

diff.c

@@ -4602,7 +4602,8 @@ void diff_setup_done(struct diff_options *options)
 	 * inside contents.
 	 */
 
-	if ((options->xdl_opts & XDF_WHITESPACE_FLAGS))
+	if ((options->xdl_opts & XDF_WHITESPACE_FLAGS) ||
+	    options->ignore_regex_nr)
 		options->flags.diff_from_contents = 1;
 	else
 		options->flags.diff_from_contents = 0;

t/t4015-diff-whitespace.sh

@@ -567,6 +567,30 @@ test_expect_success '--check and --quiet are not exclusive' '
 	git diff --check --quiet
 '
 
+test_expect_success '-w and --exit-code interact sensibly' '
+	test_when_finished "git checkout x" &&
+	{
+		test_seq 15 &&
+		echo " 16"
+	} >x &&
+	test_must_fail git diff --exit-code &&
+	git diff -w >actual &&
+	test_must_be_empty actual &&
+	git diff -w --exit-code
+'
+
+test_expect_success '-I and --exit-code interact sensibly' '
+	test_when_finished "git checkout x" &&
+	{
+		test_seq 15 &&
+		echo " 16"
+	} >x &&
+	test_must_fail git diff --exit-code &&
+	git diff -I. >actual &&
+	test_must_be_empty actual &&
+	git diff -I. --exit-code
+'
+
 test_expect_success 'check staged with no whitespace errors' '
 	echo "foo();" >x &&
 	git add x &&