Geo: Generate a repository updated event when a new merge request is created from a fork

I think this would solve the issue described in https://gitlab.com/gitlab-org/gitlab-ce/issues/50318#note_94573543. When a user creates a merge request from a new fork, I believe this happens:

1. Target project fetches the source commits in question from the source project
2. Merge request diff is generated
3. Merge request refs (e.g. refs/merge_requests/...) are created on the target project

If you don't sync the target repository after step 3, then the Geo secondary may be missing commits. Viewing a commit for that merge request will result in a 404. Also commenting on the inline diff will fail the line code validation because the commits don't exist.

Does this summarize the problem @nick.thomas @jacobvosmaer-gitlab?