t1400: provide more usual input to the command

The old version was passing (among other things)

    update SP refs/heads/c NUL NUL 0{40} NUL

to "git update-ref -z --stdin" to test whether the old-value check for
c is working.  But the <newvalue> is empty, which is a bit off the
beaten track.

So, to be sure that we are testing what we want to test, provide an
actual <newvalue> on the "update" line.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1400-update-ref.sh

@@ -912,7 +912,7 @@ test_expect_success 'stdin -z update refs works with identity updates' '
 
 test_expect_success 'stdin -z update refs fails with wrong old value' '
 	git update-ref $c $m &&
-	printf $F "update $a" "$m" "$m" "update $b" "$m" "$m" "update $c" "" "$Z" >stdin &&
+	printf $F "update $a" "$m" "$m" "update $b" "$m" "$m" "update $c" "$m" "$Z" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&
 	grep "fatal: Cannot lock the ref '"'"'$c'"'"'" err &&
 	git rev-parse $m >expect &&