Very slow for large repos

```
options := git.PushOptions{
		RemoteName: remoteName,
		Auth:       auth,
		RefSpecs: []config.RefSpec{
			config.RefSpec(fmt.Sprintf("refs/heads/%s:refs/heads/%s", hash, hash)),
		},
	}

err = repo.GitRepo.Push(&options)
```

The time for second push should be significantly less than the first push. But it seems `go-git` is pushing everything again.