Fish checkout branch - updated internal function names to checkout::file and checkout::branch

[cjappl]

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 made corresponding changes to the documentation

Description

Type of change

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

Test environment

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

[cjappl]

@wfxr the only thing for you to review is I changed the internal naming of "gcf" to "forgit::checkout::file" instead of "forgit::restore"

I updated all environment variables as well. I think this makes it clearer what is related!

[cjappl]

Closes #133 when merged

[wfxr]

@cjappl

the only thing for you to review is I changed the internal naming of "gcf" to "forgit::checkout::file" instead of "forgit::restore"

A nice improvement!