push: sanitize handling of bookmark push return value Mixing return and assignment does not make sense, let's unify this.

locarepo: remove the `push` method (API) All the logic of this function is in the exchange.push function for some time. We just stop calling `localrepo.push` in `command.push` to have access to more information. Leaving `localrepo.push` in place will let third-party extensions wrap it but it would never be called by `hg push` making the wrapping useless. Therefore, the method is removed so that third-party code fail noisily and get properly upgraded.