maxCollectionNumber must be increased even in collect-only mode

This fixed a bug that caused collect-only mode to keep overriding
collect 1 during every prune

src/duplicacy_snapshotmanager.go

@@ -2006,10 +2006,6 @@ func (manager *SnapshotManager) PruneSnapshots(selfID string, snapshotID string,
 	// deletable.
 	for _, collectionName := range collections {
 
-		if collectOnly {
-			continue
-		}
-
 		matched := collectionRegex.FindStringSubmatch(collectionName)
 		if matched == nil {
 			continue
@@ -2020,6 +2016,10 @@ func (manager *SnapshotManager) PruneSnapshots(selfID string, snapshotID string,
 			maxCollectionNumber = collectionNumber
 		}
 
+		if collectOnly {
+			continue
+		}
+
 		collectionFile := path.Join(collectionDir, collectionName)
 		manager.fileChunk.Reset(false)