Use less memory while counting lines

app/views/shared/_file_highlight.html.haml

 .file-content.code.js-syntax-highlight{ class: user_color_scheme }
   .line-numbers
     - if blob.data.present?
-      - blob.data.lines.each_index do |index|
+      - blob.data.each_line.each_with_index do |_, index|
         - offset = defined?(first_line_number) ? first_line_number : 1
         - i = index + offset
         -# We're not using `link_to` because it is too slow once we get to thousands of lines.