package gitimport (	"bytes"	"context"	"fmt"	"io"	"os/exec"	"regexp")var ValidRef = regexp.MustCompile(`^[A-Za-z0-9._/-]+$`)var ErrInvalidRef = fmt.Errorf("Invalid ref.")// verifies if a reference (hash) exists. Will error if not.func RefExists(ctx context.Context, repoPath, ref string) error {	if !ValidRef.MatchString(ref) {		return ErrInvalidRef	}	cmd := exec.CommandContext(ctx, "git", "rev-parse", "--verify", "--quiet", ref+"^{commit}")	cmd.Dir = repoPath	if err := cmd.Run(); err != nil {		return fmt.Errorf("ref %q not found: %w", ref, err)	}	return nil}// uses git's `git archive --format=zip` to create a zip archive and streams it.func ZipStream(ctx context.Context, repoPath, ref string, w io.Writer) error {	if !ValidRef.MatchString(ref) {		return ErrInvalidRef	}	cmd := exec.CommandContext(ctx, "git", "archive", "--format=zip", "--", ref+"^{commit}")	cmd.Dir = repoPath // repo itself	cmd.Stdout = w     // could be the http writer	var stderr bytes.Buffer	cmd.Stderr = &stderr	if err := cmd.Run(); err != nil {		return fmt.Errorf("git archive failed: %w: %s", err, stderr.String())	}	return nil}