exclude worktree branches

[iloveitaly]

Check list

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

Description

If a branch is used by a git worktree it cannot be switched to and is prefixed with a +. This excludes these branches from gcb to avoid erroring out when they are selected.

Related to #399

Type of change

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

Test environment

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

[Chemaclass]

As @carlfriedrich says, kudos for adding the unit test! I love seen bashunit being use and getting adoption! It feels so good helping the community. Cheers!