repoview: flag `server.view` as experimental Ideally, the non-experimental version of `experimental.extra-filter-revs` will cover the use case for `server.view=immutable` well enough than having to have this dedicated configuration. Since `server.view` is not part of any release, I would prefer to have it marked as experimental to avoid having it to support it for ever.

repoview: move subsettable in a dedicated module The dictionary got moved in `branchmap` to avoid import cycle. However, we are about to needs it in repoview too. So we introduce a now module to define that that mapping.

upgrade: support upgrade to/from zstd storage (issue6088) Now that we have an official config option for a shiny format improvement, we better make it simple to migrate to/from it.

compression: introduce an official `zstd-revlog` requirement This requirement supersedes `exp-compression-zstd`. However, we keep support for the old requirement. Strictly speaking, we do not need to add a new requirement, there are no logic change making "new" repo incompatible with mercurial client using a version that support `exp-compression-zstd`. The choice to introduce a new requirement is motivated by the following: * The previous requirement was explicitly "experimental". Using it by default could confuse users. * adding support for a hypothetical third compression engine will requires new code, and will comes with its own requirement tool. * We won't use it as the default for a while since I do not think we support zstd on all platform. I can imagine we'll gain another (unrelated but on my default) requirement by the time we turn this zstd by default.

compression: only declare revlog support for available engine Even if we know that an engine support revlog compression, the Mercurial process still won't support it if the compression engine is not available.

branchcache: lazily validate nodes in iteritems() This saves ~0.30 sec on creating a new branch on our internal repo. Differential Revision: https://phab.mercurial-scm.org/D6236

branchcache: only iterate over branches which needs to be verified Otherwise we loop over all the branches and call _verifybranch() even if not required. Differential Revision: https://phab.mercurial-scm.org/D6240

branchcache: fix the docstring of _verifybranch() Initially the function was designed to support verifying all branches but later I decided to have a separate function. I forget to remove the doc related to that. Thanks to Yuya for spotting this in review. Differential Revision: https://phab.mercurial-scm.org/D6239

branchcache: don't verify while creating a copy The copy will not be mark as verified, so there is no need to verify nodes. Thanks to Yuya who spotted this while reviewing. Differential Revision: https://phab.mercurial-scm.org/D6238

pull: improved message issued in case of failed update When running `hg pull --update`, the update step may fail. Nothing in the error message help to understand the abort is related to the secondary step (update) instead of the primary step (pull). We now add some information to the error message to clarify it comes from the update part. It is useful in various situation (uncommitted changes blocking the update, update to hidden destination, etc...) The pull output is updated from: $ hg pull ../repo-Bob --rev 956063ac4557 --update pulling from ../repo-Bob searching for changes adding changesets adding manifests adding file changes added 2 changesets with 0 changes to 2 files (+1 heads) (2 other changesets obsolete on arrival) abort: filtered revision '6'! to: $ hg pull ../repo-Bob --rev 956063ac4557 --update pulling from ../repo-Bob searching for changes adding changesets adding manifests adding file changes added 2 changesets with 0 changes to 2 files (+1 heads) (2 other changesets obsolete on arrival) abort: cannot update to target: filtered revision '6'! (I am not sure why the actual error, "filtered revision '6'", is not using the more modern format mentioning the obsolescence fate of '6')