branchmap: server should not advertise secret changeset in branchmap (Issue3303) Discovery now use an overlay above branchmap to prune invisible "secret" changeset from branchmap. To minimise impact on the code during the code freeze, this is achieve by recomputing non-secret heads on the fly when any secret changeset exists. This is a computation heavy approach similar to the one used for visible heads. But few sever should contains secret changeset anyway. See comment in code for more robust approach. On local repo the wrapper is applied explicitly while the wire-protocol take care of wrapping branchmap call in a transparent way. This could be unified by the Peter Arrenbrecht and Sune Foldager proposal of a `peer` object. An inappropriate `(+i heads)` may still appear when pushing new changes on a repository with secret changeset. (see Issue3394 for details)