GithubImporter: Optimize Pull Request MergedBy Importer

Current State

The Gitlab::GithubImport::Importer::PullRequestsMergedByImporter is skipping already imported merge requests, after fetching them from the database. Although this avoids new network requests, this still fetches more data from our database than required to do the job.

Proposal

Use the cached merge requests ids to fetch only merge requests that weren't yet imported