branching: merge default into stable for 6.8rc0

relnotes: add 6.8rc0

branch3: use an explicitely experimental name for files Since this format is still experimental, we don't want to have to side-step the `branch3` name in case people do start using it before it's stable.

obsolete: simplify relevantmarker Drop duplicate assignment from a merge failure. Save one loop iteration by exploiting that pendingnodes will be seennodes after the first round anyway, so just pre-initialize the set accordingly. From Anton Shestakov's review on !867. Performance difference for my test case is in the noise.

exchange: improve computation of relevant markers for large repos Find the candidates for nodes with relevant markers by going over all markers instead of iterating over all nodes. Most nodes will not have markers anyway. Further optimize the code by allowing revsets as well, which reduces the materialization cost.

branching: merge stable into default

narrow: add a test for linkrev computation done during widen This new tests show that the linkrev computed and sent by the server might end up being wrong during a widen operation.

obsolete: quote the feature name This makes it at least somewhat clearer that hg is talking about some specific feature and not just outdated code.

branching: merge stable into default

clonebundles: add missing newline to legacy response This seems to have been removed in 6.5 (likely by 60f9602b413e).

dirstate: remove the python-side whitelist of allowed matchers This whitelist is too permissive because it allows matchers that contain disallowed ones deep inside, for example through `intersectionmatcher`. It is also too restrictive because it doesn't pass through some of the matchers we support, such as `patternmatcher`. It's also unnecessary because unsupported matchers raise `FallbackError` and we fall back anyway. Making this change makes more of the tests use rust code path, and therefore subtly change behavior. For example, rust status in largefiles repos seems to have strange behavior.

match: make `was_tampered_with` work recursively This is useful if we are to use it outside of Rust, when deciding whether or not to do some fast-path operation with a given matcher.