t1400: provide more usual input to the command

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>