An edge case was found where a delegate attempted to merge a patch,
but had not pushed to their default branch yet.
This resulted in the `old` not being defined and the check to revert
and/or merge not happening – so the patch continued to appear as open.
Since `old` only matters for the revert and merge check for the
default branch, resolving the `old` side does its best to find a
commit for the checks.
It does this by falling back to getting the canonical head before the
push, and using that as the old.
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