Use the proper GitLab URL for links in Wiki

CHANGELOG

@@ -36,6 +36,7 @@ v 8.8.0 (unreleased)
 
 v 8.7.4
   - Fix always showing build notification message when switching between merge requests
+  - Fix links on wiki pages for relative url setups. !4026 (Artem Sidorenko)
 
 v 8.7.3
   - Emails, Gitlab::Email::Message, Gitlab::Diff, and Premailer::Adapter::Nokogiri are now instrumented

app/models/project_wiki.rb

@@ -40,7 +40,7 @@ class ProjectWiki
   end
 
   def wiki_base_path
-    ["/", @project.path_with_namespace, "/wikis"].join('')
+    [Gitlab.config.gitlab.url, "/", @project.path_with_namespace, "/wikis"].join('')
   end
 
   # Returns the Gollum::Wiki object.

lib/gitlab/url_builder.rb

@@ -62,7 +62,7 @@ module Gitlab
     end
 
     def wiki_page_url
-      "#{Gitlab.config.gitlab.url}#{object.wiki.wiki_base_path}/#{object.slug}"
+      "#{object.wiki.wiki_base_path}/#{object.slug}"
     end
   end
 end

spec/lib/gitlab/url_builder_spec.rb

@@ -112,7 +112,7 @@ describe Gitlab::UrlBuilder, lib: true do
         wiki_page = build(:wiki_page)
         url = described_class.build(wiki_page)
 
-        expect(url).to eq "#{Gitlab.config.gitlab.url}#{wiki_page.wiki.wiki_base_path}/#{wiki_page.slug}"
+        expect(url).to eq "#{Gitlab.config.gitlab.url}/#{wiki_page.wiki.project.path_with_namespace}/wikis/#{wiki_page.slug}"
       end
     end
   end

spec/models/project_wiki_spec.rb

@@ -38,7 +38,9 @@ describe ProjectWiki, models: true do
 
   describe "#wiki_base_path" do
     it "returns the wiki base path" do
-      wiki_base_path = "/#{project.path_with_namespace}/wikis"
+      gitlab_url = Gitlab.config.gitlab.url
+      wiki_base_path = "#{gitlab_url}/#{project.path_with_namespace}/wikis"
+
       expect(subject.wiki_base_path).to eq(wiki_base_path)
     end
   end