Revert "force "HEAD" for non-CI and git_upstream_merge_base for CI environment"

This reverts commit 33ac202.

Author: pietroalbini

src/build_helper/src/git.rs

@@ -1,8 +1,6 @@
 use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 
-use crate::ci::CiEnv;
-
 pub struct GitConfig<'a> {
     pub git_repository: &'a str,
     pub nightly_branch: &'a str,
@@ -116,8 +114,8 @@ fn git_upstream_merge_base(
 
 /// Searches for the nearest merge commit in the repository that also exists upstream.
 ///
-/// 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.
+/// 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.
 pub fn get_closest_merge_commit(
     git_dir: Option<&Path>,
     config: &GitConfig<'_>,
@@ -129,15 +127,7 @@ pub fn get_closest_merge_commit(
         git.current_dir(git_dir);
     }
 
-    let merge_base = {
-        if CiEnv::is_ci() {
-            git_upstream_merge_base(config, git_dir).unwrap()
-        } else {
-            // For non-CI environments, ignore rust-lang/rust upstream as it usually gets
-            // outdated very quickly.
-            "HEAD".to_string()
-        }
-    };
+    let merge_base = git_upstream_merge_base(config, git_dir).unwrap_or_else(|_| "HEAD".into());
 
     git.args([
         "rev-list",