revert accidental change in get_closest_merge_commit

Author: RalfJung

src/build_helper/src/git.rs

@@ -154,6 +154,7 @@ pub fn get_closest_merge_commit(
         "rev-list",
         &format!("--author={}", config.git_merge_commit_email),
         "-n1",
+        "--first-parent",
         &merge_base,
     ]);