Only show available cherry picks in preview

[carlfriedrich]

Check list

• [x] I have performed a self-review of my code
• [ ] I have commented my code in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation

Description

This is a follow-up to #266, which already switched to showing only available commits during cherry-picking. This patch does the same for the branch preview in _forgit_cherry_pick_from_branch.

Type of change

• [ ] Bug fix
• [x] New feature
• [ ] Refactor
• [ ] Breaking change
• [ ] Documentation change

Test environment

• Shell
  • [x] bash
  • [ ] zsh
  • [ ] fish
• OS
  • [x] Linux
  • [ ] Mac OS X
  • [ ] Windows
  • [ ] Others: