`/diff` branch mode: allow selecting the base branch

### Describe the feature or problem you'd like to solve

When using  `/diff`  in branch mode, the base branch is auto-detected as the default branch of the tracking remote (e.g.,  `origin/main` ). There's no way to override this - if your branch was cut from something other than the default branch, the diff includes noise from the parent branch rather than just your changes.

### Proposed solution

Allow specifying a base branch when using branch diff mode - e.g., a text input when selecting "Branch diff", or a parameter like  `/diff base=feature/xyz` . This would let users diff against any branch, not just the remote default.

### Example prompts or workflows

- Working on a branch cut from `feature/parent` - want to see only changes since that branch, not since `main`
- Comparing work against a release branch rather than the repo default

### Additional context

_No response_

`/diff` コマンドのブランチモードのエントリポイントから開始し、tracking remote のデフォルトブランチがどのように選択されるかを追跡します。代替のベースブランチを指定できる方法を追加し、その後、ブランチ間の diff が選択したブランチと比較され、親ブランチから継承された変更が除外されることを確認します。

索引モデルが issue の本文から書いたものです。