du summary with wildcard should report top-level summaries

If you run `du -s /tmp/*`, you'll see a summary for each matching file and directory in `/tmp`. The same with `gsutil du -s gs://bucket/*` will show only a single summary at the end. Instead, it should provide a summary for each implicit "subdirectory" and object at the root of the bucket.

Start by reproducing `gsutil du -s gs://bucket/*` and compare its output with `du -s /tmp/*`. Trace the `du` command's wildcard and summary handling; done means the command reports separate summaries for each root-level object and implicit subdirectory rather than one final total.

Written by the indexing model from the issue text.