git: use go-git instead of commandline tools

[fho]

Instead of using the git commandline tools, use https://github.com/go-git/go-git.
This way not external git binary will be required anymore to interact with git. It might also be a bit faster, then having to spawn an external tool

[fho]

As of now it is not possible to transition to go-git because of the following issues:

• When using go-git to evaluate if files are untracked, it takes in our monorepo for one of the apps 1m42s to run baur ls inputs. With baur 2.0.0 it takes 0.5s. This is a blocker. There is a related open issue in go-git: Status() is slow with a large number of untracked files go-git/go-git#181

  func (r *Repository) WithoutUntrackedFiles(paths ...string) ([]string, error) {
  	wt, err := r.repo.Worktree()
  	if err != nil {
  		return nil, err
  	}   
  	status, err := wt.Status()
  	if err != nil {
  		return nil, fmt.Errorf("fetching worktree status failed: %w", err)
  	}    
  	res := make([]string, 0, len(paths))    
  	for _, p := range paths {
  		fmt.Printf("calling IsUntracked: %q\n", p)
  		if !status.IsUntracked(p) {
  			res = append(res, p)
  		}
  	}  
  	return res, nil
  }

• According to worktree.Status() not respecting .gitignore go-git/go-git#500 go-git does not handle nested .gitignore files correctly

• Retrieving the repository status fails if the new git fsmonitor is used: Worktree.Status() always fail go-git/go-git#299