t4020: don't use grep -a

Solaris /usr/bin/grep doesn't understand "-a". In this case
we can just include the expected output with the test, which
is a better test anyway.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4020-diff-external.sh

@@ -103,7 +103,8 @@ echo NULZbetweenZwords | perl -pe 'y/Z/\000/' > file
 
 test_expect_success 'force diff with "diff"' '
 	echo >.gitattributes "file diff" &&
-	git diff | grep -a second
+	git diff >actual &&
+	test_cmp ../t4020/diff.NUL actual
 '
 
 test_done