dirstate: remove the python-side whitelist of allowed matchers
This whitelist is too permissive because it allows matchers that contain
disallowed ones deep inside, for example through `intersectionmatcher`.
It is also too restrictive because it doesn't pass through
some of the matchers we support, such as `patternmatcher`.
It's also unnecessary because unsupported matchers raise
`FallbackError` and we fall back anyway.
Making this change makes more of the tests use rust code path,
and therefore subtly change behavior. For example, rust status
in largefiles repos seems to have strange behavior.
# `rhg`
The `rhg` executable implements a subset of the functionnality of `hg`
using only Rust, to avoid the startup cost of a Python interpreter.
This subset is initially small but grows over time as `rhg` is improved.
When fallback to the Python implementation is configured (see below),
`rhg` aims to be a drop-in replacement for `hg` that should behave the same,
except that some commands run faster.
## Building
To compile `rhg`, either run `cargo build --release` from this `rust/rhg/`
directory, or run `make build-rhg` from the repository root.
The executable can then be found at `rust/target/release/rhg`.
## Mercurial configuration
`rhg` reads Mercurial configuration from the usual sources:
the user’s `~/.hgrc`, a repository’s `.hg/hgrc`, command line `--config`, etc.
It has some specific configuration in the `[rhg]` section.
See `hg help config.rhg` for details.
## Installation and configuration example
For example, to install `rhg` as `hg` for the current user with fallback to
the system-wide install of Mercurial, and allow it to run even though the
`rebase` and `absorb` extensions are enabled, on a Unix-like platform:
* Build `rhg` (see above)
* Make sure the `~/.local/bin` exists and is in `$PATH`
* From the repository root, make a symbolic link with
`ln -s rust/target/release/rhg ~/.local/bin/hg`
* Configure `~/.hgrc` with:
```
[rhg]
on-unsupported = fallback
fallback-executable = /usr/bin/hg
allowed-extensions = rebase, absorb
```
* Check that the output of running
`hg notarealsubcommand`
starts with `hg: unknown command`, which indicates fallback.
* Check that the output of running
`hg notarealsubcommand --config rhg.on-unsupported=abort`
starts with `unsupported feature:`.