changelog: parse copy metadata if available in extras This lets read back the copy metadata we just started writing. There are still many places left to teach about getting the copy information from the changeset, but we have enough ({file_copies}, specifically) that we can add it now and have some test coverage of it. Differential Revision: https://phab.mercurial-scm.org/D6186

copies: add config option for writing copy metadata to file and/or changset This introduces a config option that lets you choose to write copy metadata to the changeset extras instead of to filelog. There's also an option to write it to both places. I imagine that may possibly be useful when transitioning an existing repo. The copy metadata is stored as two fields in extras: one for copies since p1 and one for copies since p2. I may need to add more information later in order to make copy tracing faster. Specifically, I'm thinking out recording which files were added or removed so that copies._chaincopies() doesn't have to look at the manifest for that. But that would just be an optimization and that can be added once we know if it's necessary. I have also considered saving space by using replacing the destination file path by an index into the "files" list, but that can also be changed later (but before the feature is ready to release). Differential Revision: https://phab.mercurial-scm.org/D6183

revsetbenchmark: add some simpler revset for heads and roots This revset might leverage compiled code in the future so lets start to track them.

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.