Fix timeout when rendering file with thousands of lines.

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 v 7.12.0 (unreleased)
+  - Fix timeout when rendering file with thousands of lines.
   - Don't notify users mentioned in code blocks or blockquotes.
   - Omit link to generate labels if user does not have access to create them (Stan Hu)
   - Disable changing of the source branch in merge request update API (Stan Hu)

app/views/shared/_file_highlight.html.haml

@@ -4,7 +4,8 @@
       - blob.data.lines.to_a.size.times do |index|
         - offset = defined?(first_line_number) ? first_line_number : 1
         - i = index + offset
-        = link_to "#L#{i}", id: "L#{i}", rel: "#L#{i}" do
+        / We're not using `link_to` because it is too slow once we get to thousands of lines.
+        %a{href: "#L#{i}", id: "L#{i}", rel: "#L#{i}"}
           %i.fa.fa-link
           = i
   :preserve