Don't process cross-reference notes from forks
If a commit mentions an issue or merge request a cross-reference note will be created in the upstream issue/mr on push to a fork's default branch. This can lead to a lot of noise in the upstream project. There's no need to create a note every time a fork pushes a commit with a mention. Otherwise every fork that does a simple update from upstream operation will trigger the mention.
Related(ish) to https://dev.gitlab.org/gitlab/gitlabhq/issues/2640