gsutil cp command flattens subdirectories when -j option provided

The `-r` option provided here preserves the nested folder structure under `somefolder/`.

```
gsutil -m cp -r somefolder/* gs://my-bucket
```

Providing an additional `-j` option breaks this behaviour and results in a flattened output.

```
gsutil -m cp -r -j somefolder/* gs://my-bucket
```

Start by reproducing the two `gsutil -m cp -r` commands, comparing the output with and without `-j`. Trace the `cp` command's handling of recursive paths and parallel execution; done means the `-j` form preserves the nested structure under `somefolder/` just like the form without `-j`.

Written by the indexing model from the issue text.