Store the name of a project that's a root of a fork network

So we can keep showing it to a user in his project page.

app/services/projects/destroy_service.rb

@@ -22,6 +22,13 @@ module Projects
 
       Projects::UnlinkForkService.new(project, current_user).execute
 
+      # The project is not necessarily a fork, so update the fork network originating
+      # from this project
+      if fork_network = project.root_of_fork_network
+        fork_network.update(root_project: nil,
+                            deleted_root_project_name: project.full_name)
+      end
+
       attempt_destroy_transaction(project)
 
       system_hook_service.execute_hooks_for(project, :destroy)

spec/services/projects/destroy_service_spec.rb

@@ -212,6 +212,19 @@ describe Projects::DestroyService do
     end
   end
 
+  context 'as the root of a fork network' do
+    let!(:fork_network) { create(:fork_network, root_project: project) }
+
+    it 'updates the fork network with the project name' do
+      destroy_project(project, user)
+
+      fork_network.reload
+
+      expect(fork_network.deleted_root_project_name).to eq(project.full_name)
+      expect(fork_network.root_project).to be_nil
+    end
+  end
+
   def destroy_project(project, user, params = {})
     if async
       Projects::DestroyService.new(project, user, params).async_execute