Warn user about losing merge request diff

lib/tasks/migrate/migrate_mr.rake

 # This taks will reload commits/diff for all merge requests
 desc "GITLAB | Migrate Merge Requests"
 task migrate_merge_requests: :environment do
+  puts "Since 5.1 old merge request serialization logic was replaced with a better one."
+  puts "It makes old merge request diff invalid for GitLab 5.1+"
+  puts "* * *"
+  puts "This will rebuild commits/diffs info for existing merge requests."
+  puts "You will lose merge request diff if its already merged."
+  ask_to_continue
+
   MergeRequest.find_each(batch_size: 20) do |mr|
     mr.st_commits = []
     mr.save