Improve prune output/statistics

[simonsan]

And I think we should open an issue about the prune output/statistics - There it should be actually get clear why the packs are repacked, but it can be actually multiple reasons: Not fully used + too small can both apply, for example. I didn't get an idea so far how to present such kind of results in a good way...

Originally posted by @aawsome in rustic-rs/rustic#1017 (comment)