rsync-backup.in: Add double-checking for the manifest comparison.

While hacking on 1c0d861..., I temporarily introduced a bug which failed
to spot differences between the local and remote fshash(1) manifests.
Add an explicit check of file hashes as a backstop to guard against this
kind of bug now or in the future.

Also dump the hashes into the log, so that one can verify after the fact
that everything actually worked properly.