dirstate-tree: Keep a counter of descendant nodes that have an entry … and change the `DirstateMap::has_dir` method to be based on this counter being non-zero instead of the mere presence of a node. A node with zero descendent with an entry currently should be removed from the tree, but soon we’ll make the dirstate track additional nodes. (Specifically, for non-ignored directories in order to keep track of their mtime and optimize status by doing fewer `read_dir` calls.) Differential Revision: https://phab.mercurial-scm.org/D10922

dirstate: Removed unused instances of `DirsMultiset` … in Rust-backed dirstatemap. The Python class `dirstatemap` had cached properties `_dirs` and `_alldirs` that were not used for `hastrackeddir` and `hasdir` since they were redundant with corresponding fields for the Rust `DirstateMap` struct. `dirfoldmap` is modified to reuse instead the directory iterator introduced in 3b9914b28133c0918186b6e8b9e4f1916e21338d. Differential Revision: https://phab.mercurial-scm.org/D10921

dirstate-v2: Use 32-bit integers instead of 64-bit for offsets This saves 12 bytes per node. (Nodes representing files or directories.) These are offsets to other parts of the file. This would only be a limitation for a `.hg/dirstate` file larger than 4 GiB, which would only happen for a repository with dozens of millions of files and directories. Differential Revision: https://phab.mercurial-scm.org/D10920

status: Extend read_dir caching to directories with ignored files See code comments Differential Revision: https://phab.mercurial-scm.org/D10909

dirstate-v2: Drop cached read_dir results after .hgignore changes Soon we’ll want the status algorithm to be able to skip `std::fs::read_dir` in more cases, notabling when listing unknown files but not ignored files. When ignore patterns change (which we detect by their hash, added to the dirstate-v2 format in a previous changeset), a formerly-ignored file could become unknown without changing its parent directory’s modification time. Therefore we remove any directory mtime from the dirstate, effictively invalidating the existing caches. Differential Revision: https://phab.mercurial-scm.org/D10907

censor: split the core of the logic into its own function We now have a "generic" rewrite function (only able to do censoring for now) and a thin wrapper that implement the `censor` API with it. We are now ready to start incorporating strip specific changes. Differential Revision: https://phab.mercurial-scm.org/D10904

censor: migrate the logic to a set of `censor_revs` Instead of considering a special unique censored revision within the code, we consider a set of revision (currently always of size 1). This make the main code less censor-centric and prepare for the usage of a similar approach for stripping changesets. Differential Revision: https://phab.mercurial-scm.org/D10903