[Feature Request] Configure base branch for diff panel branch view in git worktrees

**Bug Description**
Feature request: In interactive mode, the diff panel's branch view (Ctrl+X B) always compares against the repository's default branch. When working in a git worktree created from a non-default branch, it would be useful to configure or select a different base branch for this comparison.

**Environment Info**
- Platform: darwin
- Terminal: xterm-256color
- Version: 2.1.270
- Feedback ID: 7e2d85a4-a055-4f5c-bb0d-98c9b3c49d68

**Errors**
```json
[]
```

Start by reproducing the interactive diff panel branch view with Ctrl+X B in a git worktree created from a non-default branch. Trace how the view chooses the repository default branch and determine where a configurable or selectable base branch belongs. Done means the branch view compares against the intended base branch when working in such a worktree.

索引モデルが issue の本文から書いたものです。