dirstate-tree: Skip readdir() in `hg status -mard` When running the status algorithm in a mode where we don’t list unknown or ignored files, all we care about are files that are listed in the dirstate. We can there for skip making expensive calls to readdir() to list the contents of filesystem directories, and instead only run stat() to get the filesystem state of files listed in the dirstate. (This state may be an error for files that don’t exist anymore on the filesystem.) On 16 CPU threads, this reduces the time spent in the `status()` function for `hg status -mard` on an old snapshot of mozilla-central from ~70ms to ~50ms. Differential Revision: https://phab.mercurial-scm.org/D10752

dirstate-v2: Parse the dirstate lazily, with copy-on-write nodes TODO: more description Differential Revision: https://phab.mercurial-scm.org/D10751

dirstate-v2: Make the dirstate bytes buffer available in more places Differential Revision: https://phab.mercurial-scm.org/D10750

dirstate-v2: Make more APIs fallible, returning Result When parsing becomes lazy, parse error will potentially happen in more places. This propagates such errors to callers. Differential Revision: https://phab.mercurial-scm.org/D10749

dirstate-v2: Add a zero-size error type for dirstate v2 parse errors This error should only happen if Mercurial is buggy or the file is corrupted. It indicates for example that: * A part of the file refers to another part, and the byte offset or item count would cause reading out of bounds, beyond the end of the file. * The byte for an entry state has an invalid value When parsing becomes lazy, many more functions will return a `Result` with this error. Making it zero-size reduces the work that the `?` operator needs to do to pass around the error value. Differential Revision: https://phab.mercurial-scm.org/D10748

dirstate-tree: Add `NodeRef` and `ChildNodesRef` enums They are used instead of `&Node` and `&ChildNodes` respectively. The `ChildNodes` type alias also becomes a similar enum. For now they only have one variant each, to be extended later. Adding enums now forces various use sites go through new methods instead of manipulating the underlying data structure directly. Differential Revision: https://phab.mercurial-scm.org/D10747

rust: Return owned instead of borrowed DirstateEntry in DirstateMap APIs This will enable the tree-based DirstateMap to not always have an actual DirstateEntry in memory for all nodes, but construct it on demand. Differential Revision: https://phab.mercurial-scm.org/D10746