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

重現 branch2 位於合併佇列中時建立 branch3 的序列，接著執行 `gh stack unstack` 並為 branch4 建立一個 PR。檢查 stack 和 PR 建立流程，以判定為什麼它會回報每個 PR 都已合併，儘管最終的 UI 看起來是正確的。完成的標準是，該序列不再產生不準確的已合併 stack 警告，也不會留下不一致的 stack 狀態。

由索引模型根據 Issue 內容生成。