Implement getting the HEAD for a ref in a branch/tag

[yiannistri]

Currently there is no way to get the HEAD for a ref in a branch/tag without first requesting the list of recent commits. Although this approach works, there are endpoints that will return that info directly.

GitHub: https://docs.github.com/en/rest/reference/git#get-a-reference
GitLab: https://docs.gitlab.com/ee/api/commits.html#get-a-single-commit