listobjectversions: Add shortcut for Veeam blocks (#10893)

Add shortcut for `APN/1.0 Veeam/1.0 Backup/10.0`

It requests unique blocks with a specific prefix. We skip 
scanning the parent directory for more objects matching the prefix.

cmd/metacache-set.go

@@ -87,6 +87,9 @@ type listPathOptions struct {
 	// This means the cache metadata will not be persisted on disk.
 	// A transient result will never be returned from the cache so knowing the list id is required.
 	Transient bool
+
+	// singleObject will assume that prefix refers to an exact single object.
+	singleObject bool
 }
 
 func init() {
@@ -542,7 +545,7 @@ func (er *erasureObjects) listPath(ctx context.Context, o listPathOptions) (entr
 		console.Printf("listPath with options: %#v\n", o)
 	}
 	// See if we have the listing stored.
-	if !o.Create {
+	if !o.Create && !o.singleObject {
 		entries, err := er.streamMetadataParts(ctx, o)
 		switch err {
 		case nil, io.EOF, context.Canceled, context.DeadlineExceeded:
@@ -662,6 +665,10 @@ func (er *erasureObjects) listPath(ctx context.Context, o listPathOptions) (entr
 
 		// Write results to disk.
 		bw := newMetacacheBlockWriter(cacheCh, func(b *metacacheBlock) error {
+			if o.singleObject {
+				// Don't save single object listings.
+				return nil
+			}
 			if debugPrint {
 				console.Println("listPath: saving block", b.n, "to", o.objectPath(b.n))
 			}