Hackbot fails to checkout a stacked patch

[suhaibmujahid]

Example: https://hackbot.moz.tools/runs/2acd3a7e-fd3d-4e49-b6c6-3873e6d1c3dc, triggered by https://phabricator.services.mozilla.com/D307836#inline-1679088

CalledProcessError: Command '['git', '-C', '/workspace/firefox', 'fetch', '--depth=2', 'origin', '69706d7a081e']' returned non-zero exit status 128.