revlog: add opener option to enable ellipsis flag processor The ellipsis flag processor can now be registered by specifying an opener option when constructing a revlog instance. This allows us to enable ellipsis flags on a per-revlog basis. Differential Revision: https://phab.mercurial-scm.org/D4647

revlog: store flag processors per revlog Previously, revlog flag processing would consult a global dict when processing flags. This was simple. But it had the undesired side-effect that any extension could load flag processors once and those flag processors would be available to any revlog that was subsequent loaded in the process. e.g. in hgweb, if the narrow extension were loaded for repo A but not repo B, repo B would be able to decode ellipsis flags even though it shouldn't be able to. Making the flag processors dict per-revlog allows us to have per-revlog controls over what flag processors are available, thus preserving desired granular access to flag processors depending on the revlog's needs. If a flag processor is globally registered, it is still globally available. So this commit should not meaningfully change behavior. Differential Revision: https://phab.mercurial-scm.org/D4646