Feature: add interactive squash selector

[sandr01d]

Check list

• 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

Adds a function for interactive git commit --squash && git rebase -i --autosquash. Because this feature is very similar to the existing _forgit_fixup function we already have, I moved most of the code from _forgit_fixup to a _forgit_edit_commit function, that can be used for both cases.
Closes #438

Type of change

• Bug fix
• New feature
• Refactor
• Breaking change
• Documentation change

Test environment

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

[wfxr]

LGTM