Use #sub instead of #gsub!. #3945

* This is because is not a good idea to modify the original lines
* Also I run into this issue https://gitlab.com/gitlab-org/gitlab_git/issues/14
  which is returning Diff Lines with the same @new_pos value.

lib/gitlab/diff/highlight.rb

@@ -73,7 +73,7 @@ module Gitlab
 
           # Only update text if line is found. This will prevent
           # issues with submodules given the line only exists in diff content.
-          line.text = highlighted_line.gsub!(/\A\s/, line_prefix).html_safe if highlighted_line
+          line.text = highlighted_line.sub(/\A\s/, line_prefix).html_safe if highlighted_line
         end
 
         @lines