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

Reproduziere die Abfolge, in der branch3 erstellt wird, während sich branch2 in der Merge-Warteschlange befindet, führe dann `gh stack unstack` aus und erstelle einen PR für branch4. Untersuche den Stack- und PR-Erstellungsablauf, um festzustellen, warum für jeden PR „gemergt“ gemeldet wird, obwohl die resultierende UI korrekt zu erscheinen scheint. Als erledigt gilt die Aufgabe, wenn die Abfolge keine ungenaue Warnung über einen gemergten Stack mehr erzeugt und den Stack-Zustand nicht inkonsistent hinterlässt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.