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).
If the file does not exist, the file will still be added and it will be created the first time we write to it.
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