rebase when lower branch was squash merged

Hello ,
was wondering if we can make gh stack rebase support the case when lower branch was squash merged (via gh merge queues for example),
otherwise with the current approach i am getting rebase conflicts

Start at the gh stack rebase entry point and reproduce the reported case with a lower branch squash-merged through a merge queue. Trace how the current rebase identifies the lower branch and define done as rebasing successfully without the conflicts described in the issue.

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