New external API method: `git_reference_listcb`

New external API method: `git_reference_listcb`

List all the references in the repository, calling a custom
callback for each one.

The listed references may be filtered by type, or using
a bitwise OR of several types. Use the magic value
`GIT_REF_LISTALL` to obtain all references, including
packed ones.

The `callback` function will be called for each of the references
in the repository, and will receive the name of the reference and
the `payload` value passed to this method.