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.

1 files changed, 18 insertions, 12 deletions

diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 14607741932..6ec66ff9afd 100644
--- a/src/bootstrap/config.rs
+++ b/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 {
@@ -1468,7 +1472,7 @@ 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/");
@@ -1476,9 +1480,10 @@ fn download_ci_rustc_commit(
     // 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`"