dirstate-v2: Write .hg/dirstate back to disk on directory cache changes Differential Revision: https://phab.mercurial-scm.org/D10827

dirstate-v2: Skip readdir in status based on directory mtime When calling `read_dir` during `status` and the directory is found to be eligible for caching (see code comments), write the directory’s mtime to the dirstate. The presence of a directory mtime in the dirstate is meaningful and indicates eligibility. When an eligible directory mtime is found in the dirstate and `stat()` shows that the mtime has not changed, `status` can skip calling `read_dir` again and instead rely on the names of child nodes in the dirstate tree. The `tempfile` crate is used to create a temporary file in order to use its modification time as "current time" with the same truncation as other files and directories would have in their own modification time. Differential Revision: https://phab.mercurial-scm.org/D10826

dirstate-v2: Allow tree nodes without an entry to store a timestamp Timestamps are stored on 96 bits: * 64 bits for the signed number of seconds since the Unix epoch * 32 bits for the nanoseconds in the `0 <= ns < 1_000_000_000` range For now timestamps are not used or set yet. Differential Revision: https://phab.mercurial-scm.org/D10825