revlog: move the details of revlog "v0" index inside revlog.utils.constants the revlog module is quite large and this kind of format information would handy for other module. So let us start to gather this information about the format in a more appropriate place. Differential Revision: https://phab.mercurial-scm.org/D10303

revlog: add some comment in the header sections We are about to add more content so let us organise the existing content first. Differential Revision: https://phab.mercurial-scm.org/D10302

store: drop the `filefilter` argument to `_walk` No code use it anywhere. Dropping it will help replacing the function with something with a more precise semantic. Differential Revision: https://phab.mercurial-scm.org/D10314

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