Fix relative URL root in wiki_base_path

This was a bug introduced in fe5533a7.

Previously we were building the path manually and had to also include
`Gitlab.config.gitlab.relative_url_root`, but since we're using route
helpers now it's already included via `default_url_options`, which
resulted in a duplicated root in the URL.

app/models/wiki.rb

@@ -205,7 +205,7 @@ class Wiki
   end
 
   def wiki_base_path
-    Gitlab.config.gitlab.relative_url_root + web_url(only_path: true).sub(%r{/#{Wiki::HOMEPAGE}\z}, '')
+    web_url(only_path: true).sub(%r{/#{Wiki::HOMEPAGE}\z}, '')
   end
 
   private

changelogs/unreleased/218823-wiki-base-path-relative-url-root.yml

+---
+title: Fix relative URL root in wiki_base_path
+merge_request: 33841
+author:
+type: fixed

spec/lib/banzai/pipeline/wiki_pipeline_spec.rb

@@ -63,7 +63,7 @@ describe Banzai::Pipeline::WikiPipeline do
       'when GitLab is hosted at a relative URL' => '/nested/relative/gitlab' }.each do |test_name, relative_url_root|
       context test_name do
         before do
-          allow(Gitlab.config.gitlab).to receive(:relative_url_root).and_return(relative_url_root)
+          allow(Rails.application.routes).to receive(:default_url_options).and_return(script_name: relative_url_root)
         end
 
         describe "linking to pages within the wiki" do

spec/support/shared_examples/models/wiki_shared_examples.rb

@@ -32,6 +32,13 @@ RSpec.shared_examples 'wiki model' do
     it 'returns the wiki base path' do
       expect(subject.wiki_base_path).to eq("#{wiki_container.web_url(only_path: true)}/-/wikis")
     end
+
+    it 'includes the relative URL root' do
+      allow(Rails.application.routes).to receive(:default_url_options).and_return(script_name: '/root')
+
+      expect(subject.wiki_base_path).to start_with('/root/')
+      expect(subject.wiki_base_path).not_to start_with('/root/root')
+    end
   end
 
   describe '#wiki' do