Allow using `download-ci-llvm = true` outside the git checkout

@bjorn3 noticed that this is already allowed today when download-llvm is disabled, but breaks with it enabled:
```
$ ./rust2/x.py build
fatal: not a git repository (or any of the parent directories): .git
thread 'main' panicked at 'command did not execute successfully: "git" "rev-list" "--author=bors@rust-lang.org" "-n1" "--first-parent" "HEAD" "--" "/home/jnelson/rust-lang/rust2/src/llvm-project" "/home/jnelson/rust-lang/rust2/src/bootstrap/download-ci-llvm-stamp" "/home/jnelson/rust-lang/rust2/src/version"
expected success, got: exit status: 128', src/bootstrap/native.rs:134:20
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

Support it too for consistency. It's unclear to me when anyone would need to use this, but @bjorn3
feels we should support it, and it's not much additional effort to get it working.

This also updates a bunch of other git commands that were similarly depending on the current directory.

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