Merge request diffs sometimes return 500

https://sentry.gitlab.net/gitlab/gitlabcom/issues/1226184/

Redis::CommandError: ERR unknown command 'memory'
  redis/client.rb:126:in `call'
    raise reply if reply.is_a?(CommandError)
  peek/views/redis_detailed.rb:10:in `call'
    super(*args, &block)
  redis.rb:3311:in `block in method_missing'
    client.call([command] + args)
  redis.rb:52:in `block in synchronize'
    mon_synchronize { yield(@client) }
  monitor.rb:235:in `mon_synchronize'
    yield
...
(151 additional frame(s) were not displayed)

Redis::CommandError: ERR unknown command 'memory'