Improve Gitlab::Git::Repository performance

These changes result in Project#empty_repo? in GitLab CE being about 4.25 - 4.5x faster than before. For the gitlab-org/gitlab-ce repository this means this method now completes in about 100 milliseconds instead of around 450 milliseconds.

GitLab CE issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/12418