feat: add support for cycling through git rebase commands

[bo5o]

Inspired by switch.vim

[CKolkey]

Super cool idea! Two notes:

1. Add a spec.
2. If the user is using the single letter version, I think it would be a good idea to keep it to a single letter.

[CKolkey]

(Don't worry about the failing SQL test - it sometimes misbehaves)

[bo5o]

Added the spec.

About expanding the single letter versions:

I did kind of like the behavior of switch.vim to expand the single letter variant of the command into the more legible longer version. I might remember 'p' being 'pick', but what was 'r' again, 'reword' or 'reset', if I then remember that 'r' is 'reword', but I want 'reset', what was that again, and so on, you know?
If you want I can change it though.

[CKolkey]

Alright, thats compelling enough for me :)