exclude upstream check from `build_helper::git::get_closest_merge_com…

…mit`

When rust-lang/rust is configured as remote, some of the git logic (for tracking changed files)
that uses `get_closest_merge_commit` starts to produce annoying results as the upstream branch becomes
outdated quickly (since it isn't updated with git pull). We can rely on `HEAD` as we specifically treat
bors commits as merge commits, which also exist on upstream. Therefore, checking the upstream branch
unnecessarily complicates things and also causes incorrect results.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/tools/build_helper/src/git.rs

@@ -96,26 +96,10 @@ pub fn updated_master_branch(
     Err("Cannot find any suitable upstream master branch".to_owned())
 }
 
-/// Finds the nearest merge commit by comparing the local `HEAD` with the upstream branch's state.
-/// To work correctly, the upstream remote must be properly configured using `git remote add <name> <url>`.
-/// In most cases `get_closest_merge_commit` is the function you are looking for as it doesn't require remote
-/// to be configured.
-fn git_upstream_merge_base(
-    config: &GitConfig<'_>,
-    git_dir: Option<&Path>,
-) -> Result<String, String> {
-    let updated_master = updated_master_branch(config, 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())
-}
-
 /// Searches for the nearest merge commit in the repository that also exists upstream.
 ///
-/// If it fails to find the upstream remote, it then looks for the most recent commit made
-/// by the merge bot by matching the author's email address with the merge bot's email.
+/// It looks for the most recent commit made by the merge bot by matching the author's email
+/// address with the merge bot's email.
 pub fn get_closest_merge_commit(
     git_dir: Option<&Path>,
     config: &GitConfig<'_>,
@@ -127,14 +111,12 @@ pub fn get_closest_merge_commit(
         git.current_dir(git_dir);
     }
 
-    let merge_base = git_upstream_merge_base(config, git_dir).unwrap_or_else(|_| "HEAD".into());
-
     git.args([
         "rev-list",
         &format!("--author={}", config.git_merge_commit_email),
         "-n1",
         "--first-parent",
-        &merge_base,
+        "HEAD",
     ]);
 
     if !target_paths.is_empty() {