* blob: added DisplayName() method to blob.Storage
* cli: added 'kopia repo sync-to <provider>' which replicates BLOBs
Usage demo: https://asciinema.org/a/352299Fixes#509
* implemented suggestion by Ciantic to fail sync if the destination repository is not compatible with the source
* cli: added 'kopia repo sync --must-exist'
This ensures that target repository is not empty, otherwise syncing to
an accidentally unmounted filesystem directory might copy everything
again.