findLandedTaskCommit only fell back to scanning all of HEAD on git-log
exceptions. When the bounded base..HEAD range is valid but empty — e.g.
the merger fast-forward-rebased the task branch and set baseCommitSha
to the new HEAD, or later commits moved HEAD past the merge — recovery
silently returned null and re-queued the merge even though the commit
had already landed. Add an empty-stdout fallback that re-scans HEAD,
with a regression test covering the FN-2221 scenario.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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