Improve statistics for `prune`

Sample:

    counting files in repo
    building new index for repo
    [0:00] 100.00%  22 / 22 packs
    repository contains 22 packs (1377 blobs) with 90.610 MiB bytes
    processed 1377 blobs: 0 duplicate blobs, 0B duplicate
    load all snapshots
    find data that is still in use for 1 snapshots
    [0:00] 100.00%  1 / 1 snapshots
    found 409 of 1377 data blobs still in use, removing 968 blobs
    will delete 10 packs and rewrite 10 packs, this frees 64.232 MiB
    creating new index
    [0:00] 100.00%  7 / 7 packs
    saved new index as df467c6e
    done

Closes #581

src/cmds/restic/cmd_prune.go

@@ -161,7 +161,8 @@ func (cmd CmdPrune) Execute(args []string) error {
 	}
 	bar.Done()
 
-	cmd.global.Verbosef("found %d of %d data blobs still in use\n", len(usedBlobs), stats.blobs)
+	cmd.global.Verbosef("found %d of %d data blobs still in use, removing %d blobs\n",
+		len(usedBlobs), stats.blobs, stats.blobs-len(usedBlobs))
 
 	// find packs that need a rewrite
 	rewritePacks := restic.NewIDSet()
@@ -176,15 +177,25 @@ func (cmd CmdPrune) Execute(args []string) error {
 		}
 	}
 
+	removeBytes := 0
+
 	// find packs that are unneeded
 	removePacks := restic.NewIDSet()
-nextPack:
 	for packID, p := range idx.Packs {
+
+		hasActiveBlob := false
 		for _, blob := range p.Entries {
 			h := restic.BlobHandle{ID: blob.ID, Type: blob.Type}
 			if usedBlobs.Has(h) {
-				continue nextPack
+				hasActiveBlob = true
+				continue
 			}
+
+			removeBytes += int(blob.Length)
+		}
+
+		if hasActiveBlob {
+			continue
 		}
 
 		removePacks.Insert(packID)
@@ -196,7 +207,8 @@ nextPack:
 		rewritePacks.Delete(packID)
 	}
 
-	cmd.global.Verbosef("will delete %d packs and rewrite %d packs\n", len(removePacks), len(rewritePacks))
+	cmd.global.Verbosef("will delete %d packs and rewrite %d packs, this frees %s\n",
+		len(removePacks), len(rewritePacks), formatBytes(uint64(removeBytes)))
 
 	err = repository.Repack(repo, rewritePacks, usedBlobs)
 	if err != nil {