Add a generic config file instance to an existing config
Note that the configuration object will free the file automatically.
Further queries on this config object will access each of the config file instances in order (instances with a higher priority level will be accessed first).
the priority level of the backend
if a config file already exists for the given priority level, replace it
0 on success, GIT_EEXISTS when adding more than one file for a given priority level (and force_replace set to 0), or error code