hidden: support passing --hidden with `serve --stdio`...
hidden: support passing --hidden with `serve --stdio`
This currently has no effect since the server code will filter the repository
again. However, it is clearer to put this change in its own changeset.
This will mostly impact ssh peers, clients being now able to pass --hidden when
spawning the server.
See the next changeset for details on how this is going to be used.