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]