compression: introduce an official `format.revlog-compression` option This option supersedes the `experiment.format.compression` option. The value currently supported are zlib (default) and zstd (if Mercurial was compiled with zstd support). The option gained an explicit reference to `revlog` since this is the target usage here. Different storage methods might require different compression strategies. In our tests, using zstd give a significant CPU usage improvement (both compression and decompressing) while keeping similar repository size. Zstd as other interresting mode (dictionnary, pre-text, etc?) that are probably worth exploring. However, just plain switching from zlib to zstd provide a large benefit.