t3404: use test_cmp_rev

t3404: use test_cmp_rev

There are a number of places where we compare two revisions with
    test $(git rev-parse rev1) = $(git rev-parse rev2)
when these fail there's no indication what has gone wrong and you need
to be running with `-x` to see where the test has failed. Lets use
test_cmp_rev instead.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>