prune: Use a single CountedBlobSet to track blobs

The set covers necessary, existing and duplicate blobs. This removes the
duplicate sets used to track whether all necessary blobs also exist.
This reduces the memory usage of prune by about 20-30%.

internal/restic/blob_set.go

@@ -31,6 +31,10 @@ func (s BlobSet) Delete(h BlobHandle) {
 	delete(s, h)
 }
 
+func (s BlobSet) Len() int {
+	return len(s)
+}
+
 // Equals returns true iff s equals other.
 func (s BlobSet) Equals(other BlobSet) bool {
 	if len(s) != len(other) {

internal/restic/find.go

@@ -15,9 +15,14 @@ type Loader interface {
 	Connections() uint
 }
 
+type findBlobSet interface {
+	Has(bh BlobHandle) bool
+	Insert(bh BlobHandle)
+}
+
 // FindUsedBlobs traverses the tree ID and adds all seen blobs (trees and data
 // blobs) to the set blobs. Already seen tree blobs will not be visited again.
-func FindUsedBlobs(ctx context.Context, repo Loader, treeIDs IDs, blobs BlobSet, p *progress.Counter) error {
+func FindUsedBlobs(ctx context.Context, repo Loader, treeIDs IDs, blobs findBlobSet, p *progress.Counter) error {
 	var lock sync.Mutex
 
 	wg, ctx := errgroup.WithContext(ctx)