diff --check: do not unconditionally complain about trailing empty lines

Recently "git diff --check" learned to detect new trailing blank lines
just like "git apply --whitespace" does.  However this check should not
trigger unconditionally.  This patch makes it honor the whitespace
settings from core.whitespace and gitattributes.

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

t/t4019-diff-wserror.sh

@@ -13,7 +13,8 @@ test_expect_success setup '
 	echo " 	HT and SP indent" >>F &&
 	echo "With trailing SP " >>F &&
 	echo "Carriage ReturnQ" | tr Q "\015" >>F &&
-	echo "No problem" >>F
+	echo "No problem" >>F &&
+	echo >>F
 
 '
 
@@ -160,4 +161,21 @@ test_expect_success 'with cr-at-eol (attribute)' '
 
 '
 
+test_expect_success 'trailing empty lines (1)' '
+
+	rm -f .gitattributes &&
+	test_must_fail git diff --check >output &&
+	grep "ends with blank lines." output &&
+	grep "trailing whitespace" output
+
+'
+
+test_expect_success 'trailing empty lines (2)' '
+
+	echo "F -whitespace" >.gitattributes &&
+	git diff --check >output &&
+	! test -s output
+
+'
+
 test_done