store: document the `walk` method Differential Revision: https://phab.mercurial-scm.org/D10313

revlog: fix error about unknown compression format in py3 In py2, the error is something like: abort: unknown compression type 'x'! In py3, we get the following unhelpful message: abort: unknown compression type <memory at 0x7f4650b5cdc8>! Switch to something like: abort: unknown compression type 78! Differential Revision: https://phab.mercurial-scm.org/D10318

revlog-compression: fix computation of engine availability We don't just need the engine to be define, we need it to be available and able to do be used for revlog compression. Without this change, `zstd` could be selected as a viable option for repository creation on platform where it is not available. Differential Revision: https://phab.mercurial-scm.org/D10325