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

Reproduce la secuencia en la que se crea branch3 mientras branch2 está en la cola de merge, luego ejecuta `gh stack unstack` y crea un PR para branch4. Inspecciona el flujo de creación del stack y del PR para determinar por qué informa de que cada PR está mergeado a pesar de que la UI resultante parece correcta. Se considera terminado cuando la secuencia ya no produce una advertencia inexacta sobre un stack mergeado ni deja el estado del stack inconsistente.

Escrito por el modelo de indexación a partir del texto del issue.