Local `git` tests will fail with `fatal: transport 'file' not allowed` when using git 2.38.1 or higher, due to a fix for `CVE-2022-39253`.
This was fixed in CI in #33429, but that doesn't help the issue for anyone's local environment. Instead of fixing this with git config in CI, we should ensure that the tests run anywhere.
- [x] Introduce `spack.util.git`.
- [x] Use `spack.util.git.get_git()` to get a git executable, instead of `which("git")` everywhere.
- [x] Make all `git` tests use a `git` fixture that goes through `spack.util.git.get_git()`.
- [x] Add `-c protocol.file.allow=always` to all `git` invocations under `pytest`.
- [x] Revert changes from #33429, which are no longer needed.
