Add a copy command to copy snapshots between repositories. It allows the user
to specify a destination repository, password, password-file, password-command
or key-hint to supply the necessary details to open the destination repository.
You need to supply a list of snapshots to copy, snapshots which already exist
in the destination repository will be skipped.
Note, when using the network this becomes rather slow, as it needs to read the
blocks, decrypt them using the source key, then encrypt them again using the
destination key before finally writing them out to the destination repository.
