stabletailgraph: omit uninteresting linear parts in test log The test uses tags for the nodes of interest. There is no need to print the linear parts which exist only to force the rank.

stabletailgraph: fix typo in test

stabletailgraph: fix terminology in doc

perf: introduce a `perf::stream-locked-section` command This command benchmark the initial part of a stream clone, where the repository is locked.

transaction: tr._names are actually bytes, use byte string to join them Looks like this is never checked in the tests? But trying e.g. `print(tr)` by hand before this change would give: TypeError: sequence item 0: expected str instance, bytes found Looks like tr._names are always bytes (although this isn't actually enforced, but maybe at some point it will at least be type checked e.g. by pytype).

nix: add nix-defined package and devel env (flake) This adds a Nix Flake which defines: - package recipies to build and run Mercurial (C, C+Rust variants), - a meta package to run the test suite in an isolated sandbox, - a development environment with some tools (notably Python and the Black formatter, pinned to match the CI). Python is pinned to the recommended version. The Rust toolchain is a fairly recent one provided by the Nixpkgs channel. It is not yet pinned to the same version as the "reference" Debian package, but this does not seem to cause any issue. Example usage of local commands are provided in the `flake.nix` file. Once merged, it should also be possible to pull and run directly from the source repository directly with something like: `nix run hg+https://foss.heptapod.net/mercurial/mercurial-devel?ref=TOPIC&dir=contrib/nix' -- version`

hgignore: ignore nix result symlink