Consolidate how Spack uses git (#34700)

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.

.github/workflows/setup_git.sh

@@ -2,10 +2,6 @@
 git config --global user.email "spack@example.com"
 git config --global user.name "Test User"
 
-# See https://github.com/git/git/security/advisories/GHSA-3wp6-j8xr-qw85 (CVE-2022-39253)
-# This is needed to let some fixture in our unit-test suite run
-git config --global protocol.file.allow always
-
 # create a local pr base branch
 if [[ -n $GITHUB_BASE_REF ]]; then
     git fetch origin "${GITHUB_BASE_REF}:${GITHUB_BASE_REF}"