fix os.copy

Author: pawelsadlo

os/src/FileOps.scala

@@ -203,7 +203,7 @@ object copy {
     def copyOne(p: Path): Unit = {
       val target = to / p.relativeTo(from)
       if (mergeFolders && isDir(p, followLinks) && !exists(target,followLinks)) {
-        // nothing to do
+        os.makeDir(target)
       } else if (mergeFolders && isDir(p, followLinks) && isDir(target, followLinks)) {
         // nothing to do
       } else {