fix download-llvm logic for subtree sync branches

Author: RalfJung

src/build_helper/src/git.rs

@@ -150,11 +150,18 @@ pub fn get_closest_merge_commit(
         }
     };
 
+    // Now that rust-lang/rust is the only repo using bors, we can search the entire
+    // history for a bors commit, not just "first parents". This is crucial to make
+    // this logic work when the user has currently checked out a subtree sync branch.
+    // At the same time, we use this logic in CI where only a tiny part of the history
+    // is even checked out, making this kind of history search very fragile. It turns
+    // out that by adding `--diff-merges=first-parent`, we get a usable reply
+    // even for sparse checkouts: it will just return the most recent bors commit.
     git.args([
         "rev-list",
         &format!("--author={}", config.git_merge_commit_email),
         "-n1",
-        "--first-parent",
+        "--diff-merges=first-parent",
         &merge_base,
     ]);