Change: Don't print skipped snapshots by default in `copy` command

The `copy` command used to print each snapshot that was skipped because it
already existed in the target repository. The amount of this output could
practically bury the list of snapshots that were actually copied.

From now on, the skipped snapshots are by default not printed at all, but
this can be re-enabled by increasing the verbosity level of the command.

https://github.com/restic/restic/issues/4033
https://github.com/restic/restic/pull/4066
