Geo: When a Snippet is not yet replicated, it should redirect to the primary

This is more a feature request than a bug, since the feature was added only for Project repos (and kind of Wikis but not really: if the Project repo doesn't exist yet, then the Wiki requests are also redirected, I think is what happens).

Example of the feature:

1. Have a Geo deployment, with a single Git URL
2. Wipe the Geo secondary data and reset the tracking database
3. During backfill, attempt to clone Snippets from the secondary
4. Many requests will fail since the repo doesn't exist yet

Basically though, we should make https://gitlab.com/gitlab-org/gitlab/-/blob/v13.6.2-ee/ee/app/controllers/ee/repositories/git_http_client_controller.rb aware of non-project-repos.