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: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -1631,7 +1631,8 @@ static void builtin_checkdiff(const char *name_a, const char *name_b,
 		ecb.priv = &data;
 		xdi_diff(&mf1, &mf2, &xpp, &xecfg, &ecb);
 
-		if (data.trailing_blanks_start) {
+		if ((data.ws_rule & WS_TRAILING_SPACE) &&
+		    data.trailing_blanks_start) {
 			fprintf(o->file, "%s:%d: ends with blank lines.\n",
 				data.filename, data.trailing_blanks_start);
 			data.status = 1; /* report errors */

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