Add menu items to move to next/previous diffs
user
```coffeescript
'menu': [
  {
    'label': 'Packages'
    'submenu': [
      'label': 'Git Diff'
      'submenu': [
        { 'label': 'Move to Next Diff', 'command': 'git-diff:move-to-next-diff' }
        { 'label': 'Move to Previous Diff', 'command': 'git-diff:move-to-previous-diff' }
      ]
    ]
  }
]
```
assistant