Enhancement: Improve `prune` in presence of duplicate blobs

Restic `prune` always used to repack all pack files containing duplicate
blobs. This effectively removed all duplicates during prune. However, one 
of the consequences was that all those pack files were downloadeded and
duplicate blobs did not contribute to the threshold for unused repository
space.
This is now changed and `prune` works nice and fast also if there are lots
of duplicates.

https://github.com/restic/restic/issues/3114
https://github.com/restic/restic/pull/3290