t1400: assert output of update-ref

t1400: assert output of update-ref

In t1400-update-ref.sh test 'transaction can create and delete' creates
files "expect" and "actual", but doesn't compare them.  Similarly, test
'transaction cannot restart ongoing transaction' redirects output of
"git update-ref" to file "actual", but doesn't check its contents with
any assertions.

Assert output of "git update-ref" in tests to improve test coverage in
t1400-update-ref.sh.

Signed-off-by: Andrei Rybak <rybak.a.v@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>