Bugfix: Correct prune statistics for partially compressed repositories

In a partially compressed repository, one data blob can exist both in an
uncompressed and a compressed version. This caused the `prune` statistics to
become inaccurate and e.g. report a too high value for the unused size, such
as "unused size after prune: 16777215.991 TiB". This has now been fixed.

https://github.com/restic/restic/issues/3918
https://github.com/restic/restic/pull/3980