It is apparently possible to add files to the git index, even if they
are part of the gitignore (see e.g.
https://stackoverflow.com/questions/45400361/why-is-gitignore-not-ignoring-my-files,
even though it's strange that the gitignore entries existed before the
files were added, i wouldn't know how to get them added in that case). I
ran
```
git rm -r --cached .
```
then change the gitignore not actually ignore those files with the
exception of
`crates/ruff_cli/resources/test/fixtures/cache_mutable/source.py`, which
is actually a generated file.
Reimplements https://github.com/astral-sh/ruff/pull/3104
Closes https://github.com/astral-sh/ruff/issues/5726
Note that we will generate the hash for a cache key twice in normal
operation. Once to check for the cached item and again to update the
cache. We could optimize this by generating the hash once in
`diagnostics::lint_file` and passing the `u64` into `get` and `update`.
We'd probably want to wrap it in a `CacheKeyHash` enum for type safety.
## Test plan
Unit tests for Windows and Unix.
Manual test with case from issue
```
❯ touch fake.py
❯ chmod +x fake.py
❯ ./target/debug/ruff --select EXE fake.py
fake.py:1:1: EXE002 The file is executable but no shebang is present
Found 1 error.
❯ chmod -x fake.py
❯ ./target/debug/ruff --select EXE fake.py
```
## Summary
And remove cached files that we haven't seen for a certain period of
time, currently 30 days.
For the last seen timestamp we actually use an `u64`, it's smaller on
disk than `SystemTime` (which size is OS dependent) and fits in an
`AtomicU64` which we can use to update it without locks.
## Test Plan
Added a new unit test, run by `cargo test`.
## Summary
Open cache files in parallel (again), brings the performance back to be roughly equal to the old implementation.
## Test Plan
Existing tests should keep working.
