Add amend commit feature

[hiro0107]

Implement amend commit feature

[ericcornelissen]

Works for me on windows 👍

I would like to see one change tho, I think the amend checkbox should be disabled by default when opening the commit dialog. But currently the checkbox stays checked after an --amend commit was made.

[hiro0107]

@ericcornelissen
Thank you for reviewing my code

I fixed the code as your request

[gforge]

Added the previous log in my fork: https://github.com/gforge/atom-git-control/commit/c0239fa305194950497d6529dcb4c97b0f534c23

This fork also contains some of the directory bug fixes