find-delta: pass the cache-delta usage policy alongside the cache-delta
The idea is to give higher level code more control to what will happens with
the cache delta passed. This should help with controling how we treat delta's
from different sources.
The final goal of this change is to allow for server modes where the client can
blindly accept any server delta without regards to any local constraints. This
will be implemented in later changesets.
# `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:`.