Reuse HTML Pipeline object in Markdown parser

The patches reduces pressure on GC and reduces markdown processing time

lib/gitlab/markdown.rb

@@ -54,7 +54,7 @@ module Gitlab
         current_user:         current_user
       )
-      pipeline = HTML::Pipeline.new(filters)
+      @pipeline ||= HTML::Pipeline.new(filters)
       context = {
         # SanitizationFilter
@@ -79,7 +79,7 @@ module Gitlab
         project_wiki:   @project_wiki
       }
-      result = pipeline.call(text, context)
+      result = @pipeline.call(text, context)
       save_options = 0
       if options[:xhtml]