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.

Author: jyn514

src/bootstrap/config.rs

@@ -1128,11 +1128,7 @@ impl Config {
             config.rust_codegen_units_std = rust.codegen_units_std.map(threads_from_config);
             config.rust_profile_use = flags.rust_profile_use.or(rust.profile_use);
             config.rust_profile_generate = flags.rust_profile_generate.or(rust.profile_generate);
-            config.download_rustc_commit = download_ci_rustc_commit(
-                &config.stage0_metadata,
-                rust.download_rustc,
-                config.verbose > 0,
-            );
+            config.download_rustc_commit = download_ci_rustc_commit(&config, rust.download_rustc);
         } else {
             config.rust_profile_use = flags.rust_profile_use;
             config.rust_profile_generate = flags.rust_profile_generate;
@@ -1304,6 +1300,15 @@ impl Config {
         config
     }
 
+    /// A git invocation which runs inside the source directory.
+    ///
+    /// Use this rather than `Command::new("git")` in order to support out-of-tree builds.
+    pub(crate) fn git(&self) -> Command {
+        let mut git = Command::new("git");
+        git.current_dir(&self.src);
+        git
+    }
+
     /// Try to find the relative path of `bindir`, otherwise return it in full.
     pub fn bindir_relative(&self) -> &Path {
         let bindir = &self.bindir;
@@ -1453,9 +1458,8 @@ fn threads_from_config(v: u32) -> u32 {
 
 /// Returns the commit to download, or `None` if we shouldn't download CI artifacts.
 fn download_ci_rustc_commit(
-    stage0_metadata: &Stage0Metadata,
+    config: &Config,
     download_rustc: Option<StringOrBool>,
-    verbose: bool,
 ) -> Option<String> {
     // If `download-rustc` is not set, default to rebuilding.
     let if_unchanged = match download_rustc {
@@ -1468,17 +1472,18 @@ fn download_ci_rustc_commit(
     };
 
     // Handle running from a directory other than the top level
-    let top_level = output(Command::new("git").args(&["rev-parse", "--show-toplevel"]));
+    let top_level = output(config.git().args(&["rev-parse", "--show-toplevel"]));
     let top_level = top_level.trim_end();
     let compiler = format!("{top_level}/compiler/");
     let library = format!("{top_level}/library/");
 
     // Look for a version to compare to based on the current commit.
     // Only commits merged by bors will have CI artifacts.
     let merge_base = output(
-        Command::new("git")
+        config
+            .git()
             .arg("rev-list")
-            .arg(format!("--author={}", stage0_metadata.config.git_merge_commit_email))
+            .arg(format!("--author={}", config.stage0_metadata.config.git_merge_commit_email))
             .args(&["-n1", "--first-parent", "HEAD"]),
     );
     let commit = merge_base.trim_end();
@@ -1491,13 +1496,14 @@ fn download_ci_rustc_commit(
     }
 
     // Warn if there were changes to the compiler or standard library since the ancestor commit.
-    let has_changes = !t!(Command::new("git")
+    let has_changes = !t!(config
+        .git()
         .args(&["diff-index", "--quiet", &commit, "--", &compiler, &library])
         .status())
     .success();
     if has_changes {
         if if_unchanged {
-            if verbose {
+            if config.verbose > 0 {
                 println!(
                     "warning: saw changes to compiler/ or library/ since {commit}; \
                           ignoring `download-rustc`"

src/bootstrap/format.rs

@@ -72,7 +72,9 @@ pub fn format(build: &Builder<'_>, check: bool, paths: &[PathBuf]) {
         Err(_) => false,
     };
     if git_available {
-        let in_working_tree = match Command::new("git")
+        let in_working_tree = match build
+            .config
+            .git()
             .arg("rev-parse")
             .arg("--is-inside-work-tree")
             .stdout(Stdio::null())
@@ -84,10 +86,7 @@ pub fn format(build: &Builder<'_>, check: bool, paths: &[PathBuf]) {
         };
         if in_working_tree {
             let untracked_paths_output = output(
-                Command::new("git")
-                    .arg("status")
-                    .arg("--porcelain")
-                    .arg("--untracked-files=normal"),
+                build.config.git().arg("status").arg("--porcelain").arg("--untracked-files=normal"),
             );
             let untracked_paths = untracked_paths_output
                 .lines()

src/bootstrap/lib.rs

@@ -634,7 +634,8 @@ impl Build {
             return;
         }
         let output = output(
-            Command::new("git")
+            self.config
+                .git()
                 .args(&["config", "--file"])
                 .arg(&self.config.src.join(".gitmodules"))
                 .args(&["--get-regexp", "path"]),
@@ -1271,12 +1272,12 @@ impl Build {
         // That's our beta number!
         // (Note that we use a `..` range, not the `...` symmetric difference.)
         let count = output(
-            Command::new("git")
+            self.config
+                .git()
                 .arg("rev-list")
                 .arg("--count")
                 .arg("--merges")
-                .arg("refs/remotes/origin/master..HEAD")
-                .current_dir(&self.src),
+                .arg("refs/remotes/origin/master..HEAD"),
         );
         let n = count.trim().parse().unwrap();
         self.prerelease_version.set(Some(n));

src/bootstrap/native.rs

@@ -118,7 +118,7 @@ pub(crate) fn maybe_download_ci_llvm(builder: &Builder<'_>) {
     if !config.llvm_from_ci {
         return;
     }
-    let mut rev_list = Command::new("git");
+    let mut rev_list = config.git();
     rev_list.args(&[
         PathBuf::from("rev-list"),
         format!("--author={}", builder.config.stage0_metadata.config.git_merge_commit_email).into(),

src/bootstrap/setup.rs

@@ -136,7 +136,7 @@ pub fn setup(config: &Config, profile: Profile) {
 
     println!();
 
-    t!(install_git_hook_maybe(&config.src));
+    t!(install_git_hook_maybe(&config));
 
     println!();
 
@@ -302,7 +302,7 @@ pub fn interactive_path() -> io::Result<Profile> {
 }
 
 // install a git hook to automatically run tidy --bless, if they want
-fn install_git_hook_maybe(src_path: &Path) -> io::Result<()> {
+fn install_git_hook_maybe(config: &Config) -> io::Result<()> {
     let mut input = String::new();
     println!(
         "Rust's CI will automatically fail if it doesn't pass `tidy`, the internal tool for ensuring code quality.
@@ -328,13 +328,12 @@ undesirable, simply delete the `pre-push` file from .git/hooks."
     };
 
     if should_install {
-        let src = src_path.join("src").join("etc").join("pre-push.sh");
-        let git = t!(Command::new("git").args(&["rev-parse", "--git-common-dir"]).output().map(
-            |output| {
+        let src = config.src.join("src").join("etc").join("pre-push.sh");
+        let git =
+            t!(config.git().args(&["rev-parse", "--git-common-dir"]).output().map(|output| {
                 assert!(output.status.success(), "failed to run `git`");
                 PathBuf::from(t!(String::from_utf8(output.stdout)).trim())
-            }
-        ));
+            }));
         let dst = git.join("hooks").join("pre-push");
         match fs::hard_link(src, &dst) {
             Err(e) => eprintln!(