Bugfix: Correctly account for duplicate data in `prune --max-unused`

`prune --max-unused size` did not correctly account for duplicate data.
If a repository contained a large amount of duplicate data, this could
previously result in pruning too little data.

This issue is now resolved.

https://github.com/restic/restic/pull/5212
https://forum.restic.net/t/restic-not-obeying-max-unused-parameter-on-prune/8879