protocol: allow hglib user to get call backs for prompts, output and errors setcbout(cbout), setcberr(cberr) and setcbprompt(cbprompt) are used to set the call back function used by the hgclient class. cb stands for call back. cbout is a function that will be called with the stdout data of the command as it runs. cbout is called with output as it is made available, which can be as partial lines or multiple lines. cberr is a function that will be called with the stderr data of the command as it runs. cberr is called with output as it is made available, which can be as partial lines or multiple lines. Command that make remote connects can prompt for username and password for HTTP/HTTPS connections. cbprompt is called when hgclient need a response to a prompt from the server. It receives the max number of bytes to return and the contents of stdout received so far. The last text sent to either cbout or cberr will contain the prompt text itself.