upgrade: clearly list optimisations This makes the command operation clearer. And this will be necessary to have a short version of this information with the next changesets, teaching `hg debugupgraderepo` about `--quiet`. Differential Revision: https://phab.mercurial-scm.org/D8428

nodemap: move the mode option to storage.revlog.nodemap.mode The option stay experimental as long as the main feature is. Differential Revision: https://phab.mercurial-scm.org/D8422

nodemap: move the option for mmap usage to storage.revlog.nodemap.mmap The option is stay experimental as long as the main feature is. Differential Revision: https://phab.mercurial-scm.org/D8421

nodemap: move and update the commend about persistence being experimental The comment was at the wrong place (on the developer option instead of the activation switch). So we move it at the right location and update it. Differential Revision: https://phab.mercurial-scm.org/D8420

nodemap: move the main switch to the `format` section The config to enable persistent nodemap is now `format.use-persistent-nodemap`. However the option remain marked as experimental because it only improve performance for people using the rust extensions. Differential Revision: https://phab.mercurial-scm.org/D8419

nodemap: drop the 'exp-' prefix for internal opener option The feature is now in a descent shape and we can consider having it "less" experimental. We won't be able to make it "totally" non-experimental, because its benefit rely on rust, which is totally experimental. Differential Revision: https://phab.mercurial-scm.org/D8418

nodemap: gate the feature behind a new requirement Now that the feature is working smoothly, a question was still open, should we gate the feature behind a new requirement or just treat it as a cache to be warmed by those who can and ignored by other. The advantage of using the cache approach is a transparent upgrade/downgrade story, making the feature easier to move to. However having out of date cache can come with a significant performance hit for process who expect an up to date cache but found none. In this case the file needs to be stored under `.hg/cache`. The "requirement" approach guarantee that the persistent nodemap is up to date. However, it comes with a less flexible activation story since an explicite upgrade is required. In this case the file can be stored in `.hg/store`. This wiki page is relevant to this questions: https://www.mercurial-scm.org/wiki/ComputedIndexPlan So which one should we take? Another element came into plan, the persistent nodemap use the `add` method of the transaction, it is used to keep track of a file content before a transaction in case we need to rollback it back. It turns out that the `transaction.add` API does not support file stored anywhere than `.hg/store`. Making it support file stored elsewhere is possible, require a change in on disk transaction format. Updating on disk file requires… introducing a new requirements. As a result, we pick the second option "gating the persistent nodemap behind a new requirements". Differential Revision: https://phab.mercurial-scm.org/D8417

nodemap: move on disk file to version 1 The current format contains the information we need, lets freeze it before the release. Differential Revision: https://phab.mercurial-scm.org/D8416

nodemap: add a new mode value, "strict" When "strict" is set, operation will refuse to use the slow path and abort if they would. This is useful for testing, benchmarking and server operation. Differential Revision: https://phab.mercurial-scm.org/D8415

nodemap: add a new mode option, with an optional "warn" value When "warn" is set, user will get notified when the slow code, used for compatibility is used. This can help people to detect situation were using that feature will give them a slowdown instead of a speedup. Differential Revision: https://phab.mercurial-scm.org/D8414