Perform a callback on each reference in the repository.
The callback function will be called for each reference in the
repository, receiving the reference object and the payload value
passed to this method. Returning a non-zero value from the callback
will terminate the iteration.
Note that the callback function is responsible to call git_reference_free
on each reference passed to it.
Additional data to pass to the callback
0 on success, non-zero callback return value, or error code