diff --git a/git_utils.py b/git_utils.py
index 5743b8fa004172778dd10c8faa4b6dcdf89e292c..b2f6bbbf59757f140f78629ded1725d4ab8a143d 100644
--- a/git_utils.py
+++ b/git_utils.py
@@ -60,7 +60,7 @@ def list_remotes(proj_path):
 
 def get_commits_ahead(proj_path, branch, base_branch):
     """Lists commits in `branch` but not `base_branch`."""
-    out = _run(['git', 'rev-list', '--left-only',
+    out = _run(['git', 'rev-list', '--left-only', '--ancestry-path',
                 '{}...{}'.format(branch, base_branch)],
                proj_path)
     return out.stdout.decode('utf-8').splitlines()