sidedata: replace sidedata upgrade mechanism with the new one Note: this is split into a separate change (like some other patches in this series) because it's not easy to have all patches work 100% and this seemed easier for reviewers. When cloning or upgrading a repo, we may need to compute (or remove) sidedata. This is the same mechanism that is used in exchange, so we re-use the new system to simplify the code and fix the remaining issues (correctly dropping flags and handling partial removal, etc.). This also highlighted an issue with `test-copies-in-changeset.t` that kept sidedata categories that are not relevant anymore. They should probably be dropped entirely, but that would be for another patch. Differential Revision: https://phab.mercurial-scm.org/D10359

sidedata: add a way of replacing an existing sidedata computer This will be useful in a future patch to replace a sequential computer with a parallel computer. We only allow for explicit replacement, to force the users to think about overriding computers. Differential Revision: https://phab.mercurial-scm.org/D10358