diff --check: do not discard error status upon seeing a good line

"git diff --check" should return non-zero when there was any whitespace
error but the code only paid attention to the error status of the last
new line in the patch.

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

t/t4017-diff-retval.sh

@@ -105,4 +105,12 @@ test_expect_success '--check with --no-pager returns 2 for dirty difference' '
 
 '
 
+
+test_expect_success 'check should test not just the last line' '
+	echo "" >>a &&
+	git --no-pager diff --check
+	test $? = 2
+
+'
+
 test_done