Enhancement: `copy` copies snapshots in batches

The `copy` command used to copy snapshots individually, even if this resulted in creating pack files
smaller than the target pack size. In particular, this resulted in many small files
when copying small incremental snapshots.

Now, `copy` copies multiple snapshots at once to avoid creating small files.

https://github.com/restic/restic/issues/5175
https://github.com/restic/restic/pull/5464