The callback settings structure
Set the callbacks to be called by the remote when informing the user about the progress of the network operations.
Textual progress from the remote. Text send over the progress side-band will be passed to this function (this is the 'counting objects' output.
Completion is called when different parts of the download process are done (currently unused).
This will be called if the remote host requires authentication in order to connect to it.
During the download of new data, this will be regularly called with the current count of progress done by the indexer.
Each time a reference is updated locally, this function will be called with information about it.
This will be passed to each of the callbacks in this struct as the last parameter.