restic: change ListBlobs to return PackBlob

PackBlob is a limited interface that only exposes a part of the
information provided by PackedBlob. Most of the changes are switches
from direct value lookups to the interface methods, with a few larger
changes to let the tests still work.

cmd/restic/cmd_stats.go

@@ -414,8 +414,8 @@ func statsDebugBlobs(ctx context.Context, repo restic.Repository) ([restic.NumBl
 		hist[i] = newSizeHistogram(2 * chunker.MaxSize)
 	}
 
-	err := repo.ListBlobs(ctx, func(pb restic.PackedBlob) {
-		hist[pb.Type].Add(uint64(pb.Length))
+	err := repo.ListBlobs(ctx, func(pb restic.PackBlob) {
+		hist[pb.Handle().Type].Add(uint64(pb.CiphertextLength()))
 	})
 
 	return hist, err