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

Riproduci la sequenza in cui branch3 viene creato mentre branch2 si trova nella coda di merge, quindi esegui `gh stack unstack` e crea una PR per branch4. Esamina il flusso dello stack e della creazione della PR per determinare perché ogni PR viene segnalata come mergiata, nonostante la UI risultante appaia corretta. Il lavoro è completato quando la sequenza non produce più un avviso inaccurato relativo a uno stack mergiato e non lascia lo stato dello stack incoerente.

Scritto dal modello di indicizzazione a partire dal testo della issue.