revlog: support none compression revlog files had uncompressed revlog entries support since forever, but it wasn't selectable or exposed explicitly. It is occassionally useful for performance testing as it avoids the latency of zlib or zstd. It also has the nice side effect of providing a non-default compression engine out-of-the-box. Differential Revision: https://phab.mercurial-scm.org/D9572