Move https://github.com/pjbgf/git2go to fluxcd/git2go

Create an official fork for git2go.

Tasks:
- [x] Copy changes from https://github.com/pjbgf/git2go
- [x] Point all references to fluxcd/git2go.
  - [x] https://github.com/fluxcd/pkg/pull/364
  - [x] https://github.com/fluxcd/source-controller/pull/919
  - [x] https://github.com/fluxcd/image-automation-controller/pull/442