Stack should be merged when merge commit is already pushed

## Reproduce step

Let's say you have this stacked PRs.

```
main <- PR#1 <- PR#2 <- PR#3
```

Now, you manually create merge commit and push it to the `main`.

```
prv_main <- PR#1 <- PR#2 <- PR#3
\---------------------+-------new_main
```

## Current Behavior

- `PR#1` is closed, because it's base is `main`, and GitHub automatically closes already merged PRs.
- `PR#2` and `PR#3` are **NOT closed**, because their bases aren't `main` (they point to previous PR's tip), so they still have diff remaining, and PR isn't closed.

## Expected Behavior

- All PRs should be closed.

## How to fix this

Stacked PR should check it's mergeability against the whole stack's base (e.g. `main` in this case).

Basically the whole point of stacked PR is that "your PR is pointing to another PR, but they are treated as pointing to base".

## Why this should be fixed

1. These PRs are shown as "open PRs" while they are already merged.
2. This will help 3rd-party integrations -- e.g. [bors](https://bors.tech/index.html) manually creates octopus merge commit of PRs and push it to main.

I know that you can manually `gh stack sync`, but I think this should be done automatically.

既存の `gh stack sync` エントリポイントと、stacked PRs がマージ済みかどうかを判定するロジックを追跡します。ここで説明されている手動で push された merge commit のシナリオと、base の扱いを比較します。完了条件は、stack の base にそれらの変更が含まれている場合に、stack 内のすべての PR がマージ済みかつクローズ済みとして認識されることです。

索引モデルが issue の本文から書いたものです。