// GetBehindAhead returns the behind/ahead commit counts information for right vs. left (both Git
// revspecs).
func (c *clientImplementor) GetBehindAhead(ctx context.Context, repo api.RepoName, left, right string) (_ *gitdomain.BehindAhead, err error) {
	ctx, _, endObservation := c.operations.getBehindAhead.With(ctx, &err, observation.Args{
		MetricLabelValues: []string{c.scope},
		Attrs: []attribute.KeyValue{
			repo.Attr(),
			attribute.String("left", left),
			attribute.String("right", right),
		},
	})
	defer endObservation(1, observation.Args{})

	if err := checkSpecArgSafety(left); err != nil {
		return nil, err
	}
	if err := checkSpecArgSafety(right); err != nil {
		return nil, err
	}

	cmd := c.gitCommand(repo, "rev-list", "--count", "--left-right", fmt.Sprintf("%s...%s", left, right))
	out, err := cmd.Output(ctx)
	if err != nil {
		return nil, err
	}
	behindAhead := strings.Split(strings.TrimSuffix(string(out), "\n"), "\t")
	b, err := strconv.ParseUint(behindAhead[0], 10, 0)
	if err != nil {
		return nil, err
	}
	a, err := strconv.ParseUint(behindAhead[1], 10, 0)
	if err != nil {
		return nil, err
	}
	return &gitdomain.BehindAhead{Behind: uint32(b), Ahead: uint32(a)}, nil
}