sync: fix cpu spinning when empty directory finding with leading slashes

Before this change the logic which makes sure we create all
directories could get confused with directories which started with
slashes and get into an infinite loop consuming 100% of the CPU.

fs/sync/sync.go

@@ -726,7 +726,7 @@ func (s *syncCopyMove) markParentNotEmpty(entry fs.DirEntry) {
 				parentDir = ""
 			}
 			delete(s.srcEmptyDirs, parentDir)
-			if parentDir == "" {
+			if parentDir == "" || parentDir == "/" {
 				break
 			}
 			parentDir = path.Dir(parentDir)