tests: demonstrate a bug with committed&ignored dirs

rust: remove out-of-date comment We've migrated to a newer version of Rust, so it doesn't make sense anymore.

rust: upgrade `rayon` dependency This includes a potential soundness fix as well as some improvements to performance which should be helpful.

rust: update zstd dependency Let's try to be the most up-to-date for this cycle. Fedora already has this version packaged, it's an added bonus.

tests: simplify a bit

dirstate-v2: fix an incorrect handling of readdir errors Make sure not to cache the results of a failed readdir call.

tests: demonstrate a bug in dirstate-v2 handling of errors

tests: add a rewriting step to detect EACCES errors

undo-files: cleanup legacy files when applicable The "journal" code is much more compact in 6.4, and so is the "undo" files as a result. However the previous version were much noisier, so let us cleanup undo files from older version too.

undo-files: clean existing files up before writing new one the in the initial design of journal/undo interaction, ages ago, new file always overwrote previous files. This is no longer the case for a long while, so it is time to properly clean things up before writing new ones. Otherwise, inconsistent "undo" state might exist on disk, leading `hg rollback` to misbehave (more that intended).