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

dirstate-tree: Downgrade `&mut Node` to `&Node` in status and serialization Mutable access is not used, and upcoming changes will make it more costly (with copy-on-write nodes that can be read from disk representation) Differential Revision: https://phab.mercurial-scm.org/D10745

dirstate-tree: Remove DirstateMap::iter_node_data_mut In an upcoming changeset we want DirstateMap to be able to work directly with nodes in their "on disk" representation, without always allocating corresponding in-memory data structures. Nodes would have two possible representations: one immutable "on disk" refering to the bytes buffer of the contents of the .hg/dirstate file, and one mutable with HashMap like the curren data structure. These nodes would have copy-on-write semantics: when an immutable node would need to be mutated, instead we allocate new mutable node for it and its ancestors. A mutable iterator of the entire tree would still be possible, but it would become much more expensive since we’d need to allocate mutable nodes for everything. Instead, remove this iterator. It was only used to clear ambiguous mtimes while serializing the `DirstateMap`. Instead clearing and serialization are now two separate passes. Clearing first uses an immutable iterator to collect the paths of nodes that need to be cleared, then accesses only those nodes mutably. Differential Revision: https://phab.mercurial-scm.org/D10744

merge with stable