Getting Every PR in this stack has already been merged, when there is a PR already

So I had branch 1 and branch 2 that created normally, and then branch3 was created while branch2 was in merge queue.

Then the git diff went weird but I fixed the merge conflicts, and thought maybe I'd try to `gh stack unstack`, but it didn't allow me because it was still part of the stack. Oh well, I continued on and tried making a PR for `branch4` and got this.

```
⚠ Every PR in this stack has already been merged on GitHub
Adding to a fully merged stack isn't supported — starting a new stack for your 2 unmerged branches based on master
Pushing to origin...
PR #52141 for branch3
✓ Created PR #52148 for branch4
✓ Stack created on GitHub with 2 PRs (stack #52149)
✓ Pushed and synced 2 branches
```

Everything looks good on the UI though

Reproduza a sequência em que branch3 é criado enquanto branch2 está na fila de merge, depois execute `gh stack unstack` e crie um PR para branch4. Inspecione o fluxo de criação do stack e do PR para determinar por que ele informa que todos os PRs foram mergeados, apesar de a UI resultante parecer correta. Considera-se concluído quando a sequência não produzir mais um aviso impreciso de stack mergeado nem deixar o estado do stack inconsistente.

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