sidedata: apply basic but tight security around exchange We don't currently have code to deal with exchange between repository using sidedata and repository not using sidedata. Until we implement such code (eg: dropping side data when pushing to a non-sidedata repo) we prevent the two kind of repo to speak to each other. This is somewhere similar to what 'treemanifest' does. Note that sidedata exchange is broken unless one use changegroup v3 anyway. See next changeset for details. Differential Revision: https://phab.mercurial-scm.org/D6939