You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: gitops.go
    path of file: ./repos/portainer/api/stacks/stackutils
    
    the code of the file until where you have to start completion: package stackutils

import (
	"fmt"

	"github.com/pkg/errors"
	portainer "github.com/portainer/portainer/api"
	"github.com/portainer/portainer/api/git"
	gittypes "github.com/portainer/portainer/api/git/types"
)

var (
	ErrStackAlreadyExists     = errors.New("A stack already exists with this name")
	ErrWebhookIDAlreadyExists = errors.New("A webhook ID already exists")
)

// DownloadGitRepository downloads the target git repository on the disk
// The first return value represents the commit hash of the downloaded git repository
func DownloadGitRepository(config gittypes.RepoConfig, gitService portainer.GitService, getProjectPath func() string) (string, error) {
	username := ""
    
func DownloadGitRepository(config gittypes.RepoConfig, gitService portainer.GitService, getProjectPath func() string) (string, error) {
	username := ""
	password := ""
	if config.Authentication != nil {
		username = config.Authentication.Username
		password = config.Authentication.Password
	}

	projectPath := getProjectPath()
	err := gitService.CloneRepository(projectPath, config.URL, config.ReferenceName, username, password, config.TLSSkipVerify)
	if err != nil {
		if errors.Is(err, gittypes.ErrAuthenticationFailure) {
			newErr := git.ErrInvalidGitCredential
			return "", newErr
		}

		newErr := fmt.Errorf("unable to clone git repository: %w", err)
		return "", newErr
	}

	commitID, err := gitService.LatestCommitID(config.URL, config.ReferenceName, username, password, config.TLSSkipVerify)
	if err != nil {
		newErr := fmt.Errorf("unable to fetch git repository id: %w", err)
		return "", newErr
	}
	return commitID, nil
}