commands: support creating stream clone bundles Now that we have support for recognizing the streaming clone bundle type, add a debug command for creating them. I decided to create a new debug command instead of adding support to `hg bundle` because stream clone bundles are not exactly used the same way as normal bundle files and I don't want to commit to supporting them through the official `hg bundle` command forever. A debug command, however, can be changed without as much concern for backwards compatibility. As part of this, `hg bundle` will explicitly reject requests to produce stream bundles. This command will be required by server operators using stream clone bundles with the clone bundles feature.