The setup job checks out with fetch-depth: 0, then refetched the base branch with
--depth=1 before calling merge-base. A depth-limited fetch into a complete clone
writes .git/shallow and grafts the fetched tip as parentless, so merge-base finds
no common commit once the base branch has moved past the pull request merge
commit. The step then failed under set -e with a bare exit 1 and no message.
Use the origin/<base> ref the checkout already provides.
Thomas Göttgens
eb6df1c649