Build a single-level focused config object from a multi-level one.
The returned config object can be used to perform get/set/delete operations on a single specific level.
Getting several times the same level from the same parent multi-level config will return different config instances, but containing the same config_file instance.
Configuration level to search for
0, GIT_ENOTFOUND if the passed level cannot be found in the multi-level parent config, or an error code