Bugfix: Fix duplicate data handling in `prune --max-unused`

The `prune --max-unused size` command 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 has now been fixed.

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