changegroup: document that 'source' parameter exists for extensions
The 'source' parameter passed to generatefiles() is unused by the
method itself, but Durham says it is used by an extension.
changegroup: removed unused 'source' parameter from prune()
The parameter has been unused since it was introduced in
6ea1f858efd9
(bundle: refactor changegroup prune to be its own function,
2013-05-30), and Durham says it is not used by his extension either.