readme: add a changelog entry about the more efficient obscache

obscache: extract a data agnostic class We now have an independent class that we can reuse for other purpose (eg: obshashrange cache)

obscache: extract the actual data update in a dedicated function This will help extract a data agnostic class.

obscache: move an assert to a lower level

obscache: use 'nullid' as the hash of an empty obsstore This align this result with what we use for 'emptykey'

obcache: move empty on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obcache: move _checkkey on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obcache: move updateneeded on the class We'll extract a smaller data agnostic class but we need to gather all method on it first.

obshistory: only display each revision once in debugobshistory When using a revision range, the same precursor could have been displayed more than once. Use a cache to display each revision only once. It's a fix until we found the nicest way to display obs history.

obshistory: display a message when one marker node has no change ctx When exchanging obs markers, there is some change contexts referencing change contexts that are not available locally. As we cannot display informations about them, instead print a message saying so.

ui: hg topic now display if current revision is in bad state (issue5533) Previously, hg topic didn't showed the state of the current revision. Now if show both informations.

obsstore: also guard agains changing object in '_checkkey' The 'upgradeneeded' function needs to be able to pass the object it fetched once.

obscache: guard from changing changelog or obsstore object We access these object once to make sure they will never change through the function (and their content will be fixed).

obscache: return the new data along-side the upgrade needs (and cache key) Having one function returning consistent data will help prevent race.

obscache: use the smaller scope function This make the function simpler and allow larger update on the 'upgradeneeded' one.

obscache: extract the cache key validation in its own function This help keep the "up to date logic" simpler and prepare a more complex version of the "upgradeneeded" logic that would return the full data needed for the upgrade.