Auto merge of #114345 - lqd:revert-113588, r=RalfJung

Revert #113588 to fix bootstrap timings

This reverts #113588 which seems to have broken perf's bootstrap timings via some git issue

https://github.com/rust-lang/rust/pull/114318#issuecomment-1660807886 show a newly broken benchmark, the error at the time was

```
fatal: Path 'src/ci/channel' exists on disk, but not in 'e62323df22ecf9c163023132d17b7114f68b72e8'.
       thread 'main' panicked at 'command did not execute successfully: cd "/home/collector/rustc-perf/rust" && "git" "show" "e62323df22ecf9c163023132d17b7114f68b72e8:src/ci/channel"
       expected success, got: exit status: 128', config.rs:1786:27
```

If this lands, it will reopen #101907 and annoy miri, but it could actually be an issue that would appear during the next bootstrap bump, not just rustc-perf today.

r? `@ghost`

2 files changed, 19 insertions, 28 deletions

diff --git a/src/bootstrap/llvm.rs b/src/bootstrap/llvm.rs
index d8718d26848..81b88d5de98 100644
--- a/src/bootstrap/llvm.rs
+++ b/src/bootstrap/llvm.rs
@@ -24,7 +24,6 @@ use crate::util::{self, exe, output, t, up_to_date};
 use crate::{CLang, GitRepo, Kind};
 
 use build_helper::ci::CiEnv;
-use build_helper::git::get_git_merge_base;
 
 #[derive(Clone)]
 pub struct LlvmResult {
@@ -129,19 +128,13 @@ pub fn prebuilt_llvm_config(
 /// This retrieves the LLVM sha we *want* to use, according to git history.
 pub(crate) fn detect_llvm_sha(config: &Config, is_git: bool) -> String {
     let llvm_sha = if is_git {
-        // We proceed in 2 steps. First we get the closest commit that is actually upstream. Then we
-        // walk back further to the last bors merge commit that actually changed LLVM. The first
-        // step will fail on CI because only the `auto` branch exists; we just fall back to `HEAD`
-        // in that case.
-        let closest_upstream =
-            get_git_merge_base(Some(&config.src)).unwrap_or_else(|_| "HEAD".into());
         let mut rev_list = config.git();
         rev_list.args(&[
             PathBuf::from("rev-list"),
             format!("--author={}", config.stage0_metadata.config.git_merge_commit_email).into(),
             "-n1".into(),
             "--first-parent".into(),
-            closest_upstream.into(),
+            "HEAD".into(),
             "--".into(),
             config.src.join("src/llvm-project"),
             config.src.join("src/bootstrap/download-ci-llvm-stamp"),
diff --git a/src/tools/build_helper/src/git.rs b/src/tools/build_helper/src/git.rs
index f20b7a2b4d7..66876e02c19 100644
--- a/src/tools/build_helper/src/git.rs
+++ b/src/tools/build_helper/src/git.rs
@@ -78,22 +78,13 @@ pub fn rev_exists(rev: &str, git_dir: Option<&Path>) -> Result<bool, String> {
 /// We will then fall back to origin/master in the hope that at least this exists.
 pub fn updated_master_branch(git_dir: Option<&Path>) -> Result<String, String> {
     let upstream_remote = get_rust_lang_rust_remote(git_dir)?;
-    for upstream_master in [format!("{upstream_remote}/master"), format!("origin/master")] {
-        if rev_exists(&upstream_master, git_dir)? {
-            return Ok(upstream_master);
-        }
+    let upstream_master = format!("{upstream_remote}/master");
+    if rev_exists(&upstream_master, git_dir)? {
+        return Ok(upstream_master);
     }
 
-    Err(format!("Cannot find any suitable upstream master branch"))
-}
-
-pub fn get_git_merge_base(git_dir: Option<&Path>) -> Result<String, String> {
-    let updated_master = updated_master_branch(git_dir)?;
-    let mut git = Command::new("git");
-    if let Some(git_dir) = git_dir {
-        git.current_dir(git_dir);
-    }
-    Ok(output_result(git.arg("merge-base").arg(&updated_master).arg("HEAD"))?.trim().to_owned())
+    // We could implement smarter logic here in the future.
+    Ok("origin/master".into())
 }
 
 /// Returns the files that have been modified in the current branch compared to the master branch.
@@ -103,13 +94,20 @@ pub fn get_git_modified_files(
     git_dir: Option<&Path>,
     extensions: &Vec<&str>,
 ) -> Result<Option<Vec<String>>, String> {
-    let merge_base = get_git_merge_base(git_dir)?;
+    let Ok(updated_master) = updated_master_branch(git_dir) else {
+        return Ok(None);
+    };
 
-    let mut git = Command::new("git");
-    if let Some(git_dir) = git_dir {
-        git.current_dir(git_dir);
-    }
-    let files = output_result(git.args(["diff-index", "--name-only", merge_base.trim()]))?
+    let git = || {
+        let mut git = Command::new("git");
+        if let Some(git_dir) = git_dir {
+            git.current_dir(git_dir);
+        }
+        git
+    };
+
+    let merge_base = output_result(git().arg("merge-base").arg(&updated_master).arg("HEAD"))?;
+    let files = output_result(git().arg("diff-index").arg("--name-only").arg(merge_base.trim()))?
         .lines()
         .map(|s| s.trim().to_owned())
         .filter(|f| {