Enhancement: `copy` copies snapshots in batches

The `copy` command used to copy snapshots one at a time, even when that
produced pack files smaller than the target pack size. That led to many small
files when copying small incremental snapshots.

The `copy` command now copies multiple snapshots together so small pack files
are avoided where possible.

https://github.com/restic/restic/issues/5453
https://github.com/restic/restic/pull/5472
