Move pre_process into render_result

The method Banzai::Renderer.pre_process would always be called,
regardless of whether the Markdown to render was already cached or not.
In cache the document _was_ cached the output of the pre-processing
pipeline was ignored resulting in it doing nothing but wasting CPU
cycles.

This commit moves Banzai::Renderer.pre_process into
Banzai::Renderer.render_result so that it's _only_ used when needed.

CHANGELOG

@@ -14,6 +14,7 @@ v 8.9.0 (unreleased)
   - Fix issue with arrow keys not working in search autocomplete dropdown
   - Fix an issue where note polling stopped working if a window was in the
     background during a refresh.
+  - Pre-processing Markdown now only happens when needed
   - Make EmailsOnPushWorker use Sidekiq mailers queue
   - Redesign all Devise emails. !4297
   - Don't show 'Leave Project' to group members

app/helpers/gitlab_markdown_helper.rb

@@ -50,8 +50,6 @@ module GitlabMarkdownHelper
 
     context[:project] ||= @project
 
-    text = Banzai.pre_process(text, context)
-
     html = Banzai.render(text, context)
 
     context.merge!(

lib/banzai.rb

@@ -7,10 +7,6 @@ module Banzai
     Renderer.render_result(text, context)
   end
 
-  def self.pre_process(text, context)
-    Renderer.pre_process(text, context)
-  end
-
   def self.post_process(html, context)
     Renderer.post_process(html, context)
   end

lib/banzai/renderer.rb

@@ -30,15 +30,11 @@ module Banzai
     end
 
     def self.render_result(text, context = {})
+      text = Pipeline[:pre_process].to_html(text, context) if text
+
       Pipeline[context[:pipeline]].call(text, context)
     end
 
-    def self.pre_process(text, context)
-      pipeline = Pipeline[:pre_process]
-
-      pipeline.to_html(text, context)
-    end
-
     # Perform post-processing on an HTML String
     #
     # This method is used to perform state-dependent changes to a String of