Skip merge flow when the upstream latest tag is already merged on downstream

This can be determined by calling the following [api](https://docs.github.com/en/rest/reference/repos#compare-two-commits),

`GET /repos/{owner}/{repo}/compare/{basehead}`

e.g. openshift/thanos:master vs thanos-io/thanos:v0.22.0
```sh
curl -s https://api.github.com/repos/openshift/thanos/compare/{master}...{thanos-io:v0.22.0} -o - | jq .ahead_by
```