Add an on-disk config file instance to an existing config
The on-disk file pointed at by path
will be opened and
parsed; it's expected to be a native Git config file following
the default Git config syntax (see man git-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).
path to the configuration file to add
replace config file at the given priority level
0 on success, GIT_EEXISTS when adding more than one file for a given priority level (and force_replace set to 0), GIT_ENOTFOUND when the file doesn't exist or error code