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

重现 branch2 处于合并队列中时创建 branch3 的序列，然后运行 `gh stack unstack` 并为 branch4 创建一个 PR。检查 stack 和 PR 创建流程，以确定为什么它会报告每个 PR 都已合并，尽管最终的 UI 看起来是正确的。完成的标准是，该序列不再产生不准确的已合并 stack 警告，也不会留下不一致的 stack 状态。

由索引模型根据 Issue 内容生成。