Fix tests under GETTEXT_POISON on relative dates

Use the i18n-specific test_i18ncmp in t/t0006-date.sh for relative dates
tests. This issue was was introduced in v1.7.10-230-g7d29a:

    7d29a i18n: mark relative dates for translation

and been broken under GETTEXT_POISON=YesPlease since.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0006-date.sh

@@ -11,7 +11,7 @@ check_show() {
 	echo "$t -> $2" >expect
 	test_expect_${3:-success} "relative date ($2)" "
 	test-date show $t >actual &&
-	test_cmp expect actual
+	test_i18ncmp expect actual
 	"
 }