phase: add an archived phase This phase allows for hidden changesets in the "user space". It differs from the "internal" phase which is intended for internal by-product only. There have been discussions at the 4.8 sprint to use such phase to speedup cleanup after history rewriting operation. Shipping it in the same release as the 'internal-phase' groups the associated `requires` entry. The important bit is to have support for this phase in the earliest version of mercurial possible. Adding the UI to manipulate this new phase later seems fine. The current plan for archived usage and user interface are as follow. On a repository with internal-phase on and evolution off: * history rewriting command set rewritten changeset in the archived phase. (This mean updating the cleanupnodes method). * keep `hg unbundle .hg/strip-backup/X.hg` as a way to restore changeset for now (backup bundle need to contains phase data) * [maybe] add a `hg strip --soft` advance flag (a light way to expose the feature without getting in the way of a better UI) Mercurial 4.8 freeze is too close to get the above in by then. We don't introduce a new repository `requirement` as we reuse the one introduced with the 'archived' phase during the 4.8 cycle.