Keep on getting `invalid changes in the merge commit`

In github.com/lemonade-sdk/lemonade we use squash merging and also use a merge queue. In experimenting with stacks we keep running into this situation:

Pretty much we queue up the entire stack, one layer merges and the rest are pulled. So then we have to monitor the next layer, requeue all of them etc.

The entire stack that showed this problem is here:
https://github.com/lemonade-sdk/lemonade/pull/3584

You can see the earlier layers how many times they got pulled out of the merge queue because of "invalid changes in the merge commit".

Start by reproducing the stacked-PR scenario described with the linked lemonade pull request and examine how gh-stack interacts with GitHub's merge queue. Done means a queued stack can advance after one layer merges without later layers being repeatedly removed for "invalid changes in the merge commit."

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