GitHub importer fails when source repository (forked) was deleted

This issue is partially related to https://gitlab.com/gitlab-org/gitlab-ce/issues/29093. See client issue for more context / additional details: https://gitlab.zendesk.com/agent/tickets/67647

How to reproduce

1. Fork a repository
2. Push some code
3. Create a merge request
4. Delete the Forked repository

Details

You can see a working example here: https://github.com/xiaogangwen/my-awesome-project/pull/11 The relevant information is:

xgwennt wants to merge 2 commits into xiaogangwen:master from unknown repository

As the source repository was deleted, it now displays unknown repository, but we can retrieve the original "label" from the API request. At the API request level, the head.repo is now nil and because of that the import_error displays the cryptic:

undefined method `id' for nil:NilClass

This is being triggered by the cross_project? in Gitlab::GithubImporter::PullRequestFormatter. It's not the only place that needs to be fixed. Basically anywhere that relies on the existence of a repository should.

How to fix

There are two possible solutions I discussed with @jameslopez. One is to check for the existence of the repository and add some extra if/elses. The other is to come up with a NullObject with some extra context to try to reply meaninfull data so we don't have to refactor everywhere.

cc @dbalexandre @balameb @DouweM @xiaogang_gitlab