rhg: faster hg cat when many files are requested With this patch I'm seeing a ~39ms improvement (220ms -> 181ms) when running [hg cat] on ~220 files in a ~260k-file repo. The timing for [hg cat] on an individual file becomes slightly worse (losing 5ms: 145ms -> 150ms). A follow-up commit is intended to improve that. Differential Revision: https://phab.mercurial-scm.org/D11615

dirstate-v2: Add support when Rust is not enabled This wires into `dirstatemap` the parser and serializer added in previous changesets. The memory representation is still the same, with a flat `dict` for `DirstateItem`s and another one for copy sources. Serialization always creates a new dirstate-v2 data file and does not support (when Rust is not enabled) appending to an existing one, since we don’t keep track of which tree nodes are new or modified. Instead the tree is reconstructed during serialization. Differential Revision: https://phab.mercurial-scm.org/D11520

dirstate-v2: Initial Python serializer This adds code seralizing a `map` and `copy_map` dicts into dirstate-v2 file formate. This is not used yet. Differential Revision: https://phab.mercurial-scm.org/D11519

dirstate-v2: initial Python parser The dirstate-v2 file format should be supported even if Rust extensions are not enabled. This changeset adds parsing code that is not used yet. Differential Revision: https://phab.mercurial-scm.org/D11518

dirstate: Move more methods to the _dirstatemapcommon base class This reduces duplication slightly and will help with supporting dirstate-v2 when Rust is not enabled. Differential Revision: https://phab.mercurial-scm.org/D11621

dirstate-v2: Add storage space for nanoseconds precision in file mtimes For now the sub-second component is always set to zero for tracked files and symlinks. (The mtime of directories for the `readdir`-skipping optimization is a different code path and already uses the full precision available.) This extra storage uses the space previously freed by replacing the 32-bit `mode` field by two bits in the existing `flags` field, so the overall size of nodes is unchanged. (This space had been left as padding for this purpose.) Also move things around in the node layout and documentation to have less duplication. Now that they have the same representation, directory mtime and file mtime are kept in the same field. (Only either one can exist for a given node.) Differential Revision: https://phab.mercurial-scm.org/D11655

status: Extract TruncatedTimestamp from fs::Metadata without SystemTime On Unix, the Rust standard library exposes `mtime` and `mtime_nsec` methods for `std::fs::Metada` whih is exactly what we need to construct a `TruncatedTimestamp`. This skips the computation in the conversion through `SystemTime` and `Result<Duration, Duration>`. Differential Revision: https://phab.mercurial-scm.org/D11654