Add empty state messages for checkout commands

[Chemaclass]

Check list

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

Description

Closes: https://github.com/wfxr/forgit/issues/475

Show informative messages when there is nothing to checkout:

• checkout_file: "Nothing to checkout." when no modified files
• checkout_tag: "Nothing to checkout: there are no tags."

Type of change

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

Test environment

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

[sandr01d]

Thanks for the update @Chemaclass. I've merged #480, could you please rebase this PR so the CI can run successfully?

[Chemaclass]

@sandr01d rebase done!

[sandr01d]

Thanks. I think you might have missed my earlier comment. Once that's done, you have my approval :slightly_smiling_face:

[sandr01d]

Thanks for your contribution @Chemaclass!