Identify if `push` event is for a pull request merge without additional API calls

[adityasaky]

Currently, the push event handler creates an RSL entry for the reference in question. As GitHub sends a push event and a pull request event when a PR is merged, the push event handler uses the API to identify if the pushed commit is part of any pull requests that were merged. If so, an RSL entry is not created because the pull request handler takes care of it instead. This issue is to discuss if we can achieve this without the additional API call, especially since the API response may not indicate that the PR was merged (there may be a delay before the change in status is reflected in the API response).