MinGW: Skip test redirecting to fd 4

... because that does not work in MinGW.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0081-line-buffer.sh

@@ -29,7 +29,7 @@ test_expect_success '0-length read, send along greeting' '
 	test_cmp expect actual
 '
 
-test_expect_success 'read from file descriptor' '
+test_expect_success NOT_MINGW 'read from file descriptor' '
 	rm -f input &&
 	echo hello >expect &&
 	echo hello >input &&