Improve performance of SyntaxHighlightFilter

By using Rouge::Lexer.find instead of find_fancy() and memoizing the
HTML formatter we can speed up the highlighting process by between 1.7
and 1.8 times (at least when measured using synthetic benchmarks). To
measure this I used the following benchmark:

    require 'benchmark/ips'

    input = ''

    Dir['./app/controllers/**/*.rb'].each do |controller|
      input << <<-EOF
      <pre><code class="ruby">#{File.read(controller).strip}</code></pre>

      EOF
    end

    document = Nokogiri::HTML.fragment(input)
    filter = Banzai::Filter::SyntaxHighlightFilter.new(document)

    puts "Input size: #{(input.bytesize.to_f / 1024).round(2)} KB"

    Benchmark.ips do |bench|
      bench.report 'call' do
        filter.call
      end
    end

This benchmark produces 250 KB of input. Before these changes the timing
output would be as follows:

    Calculating -------------------------------------
                    call     1.000  i/100ms
    -------------------------------------------------
                    call     22.439  (±35.7%) i/s -     93.000

After these changes the output instead is as follows:

Calculating -------------------------------------
                call     1.000  i/100ms
-------------------------------------------------
                call     41.283  (±38.8%) i/s -    148.000

Note that due to the fairly high standard deviation and this being a
synthetic benchmark it's entirely possible the real-world improvements
are smaller.

CHANGELOG

@@ -15,6 +15,7 @@ v 8.11.0 (unreleased)
   - Limit git rev-list output count to one in forced push check
   - Clean up unused routes (Josef Strzibny)
   - Add green outline to New Branch button. !5447 (winniehell)
+  - Improve performance of syntax highlighting Markdown code blocks
   - Update to gitlab_git 10.4.1 and take advantage of preserved Ref objects
   - Retrieve rendered HTML from cache in one request
   - Fix renaming repository when name contains invalid chararacters under project settings

lib/banzai/filter/syntax_highlight_filter.rb

@@ -17,15 +17,12 @@ module Banzai
 
       def highlight_node(node)
         language = node.attr('class')
-        code     = node.text
-
+        code = node.text
         css_classes = "code highlight"
-
-        lexer = Rouge::Lexer.find_fancy(language) || Rouge::Lexers::PlainText
-        formatter = Rouge::Formatters::HTML.new
+        lexer = lexer_for(language)
 
         begin
-          code = formatter.format(lexer.lex(code))
+          code = format(lex(lexer, code))
 
           css_classes << " js-syntax-highlight #{lexer.tag}"
         rescue
@@ -41,14 +38,27 @@ module Banzai
 
       private
 
+      # Separate method so it can be instrumented.
+      def lex(lexer, code)
+        lexer.lex(code)
+      end
+
+      def format(tokens)
+        rouge_formatter.format(tokens)
+      end
+
+      def lexer_for(language)
+        (Rouge::Lexer.find(language) || Rouge::Lexers::PlainText).new
+      end
+
       def replace_parent_pre_element(node, highlighted)
         # Replace the parent `pre` element with the entire highlighted block
         node.parent.replace(highlighted)
       end
 
       # Override Rouge::Plugins::Redcarpet#rouge_formatter
-      def rouge_formatter(lexer)
-        Rouge::Formatters::HTML.new
+      def rouge_formatter(lexer = nil)
+        @rouge_formatter ||= Rouge::Formatters::HTML.new
       end
     end
   end