Stop a project being returned as a fork of itself

What does this MR do and why?

Restores forking for users whose only forkable namespace is their personal namespace. The Fork button on a project page now offers a new fork instead of linking back to the project the user is already on.

Problem

Currently on master, such a user cannot fork a project from its page once they have deleted their own fork of it, because the Fork button links back to that same page while the forks counter shows 0. A fork network keeps its root project as a member, and find_forks_in matches any network project in the namespace without excluding the one being asked about, so Namespace#find_fork_of can answer with that project itself. The same wrong answer sends "Go to your fork" to the source project while the fork still exists.

Review notes

• Why fix find_fork_of and not find_forks_in? find_forks_in answers which of the given projects are in the network, and the root is one of them. find_fork_of answers which project in this namespace is a fork of another, where the project is never a valid answer.
• What do the query plans say? Only the subquery picking the candidate projects changes, by one != predicate. Locally it lands as a filter on the existing index-only scan over index_projects_on_namespace_id_and_id, with the same plan shape and the same cost estimate, so no new index. I have no Database Lab access as an external contributor, so please confirm that against production-sized data.
• What is left alone? A fork marked for deletion still counts, so the button points at the renamed ...-deletion_scheduled-<id> project. Same cause, but the user can still restore that project, which makes it a product question rather than a defect. Tracked at #369498 (closed).

🛠️ with ❤️ at Siemens

References

• #629277 (closed) — the issue this MR closes, with a screen recording
• #329755 (closed) — same symptom in 2021, closed as stale
• #375555 (closed) — names the wide fork network search, closed as stale
• #422242 (closed) — same symptom, closed by the reporter, confirmed by a second user

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Sign in as a user who belongs to no group that allows project creation.
2. Create a public project in their personal namespace, then fork it into that same namespace.
3. Delete the fork.
4. Open the source project page. The Fork button links to /-/forks/new instead of back to the project.

MR acceptance checklist

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