so now you have two coders looking at every line of code?
Yeah...because this is how it's done when it's done professionally. You have one coder...the guy who wrote the change...and then another coder...the one who tests it.
This happens in non-code places too, like journalism. One person writes the article, and another proofreads it. (Due to the acceleration of the news cycle, this has been going away...with predictably-bad results.) Consulting? Yes, you have quality control (another person reading and checking the deliverable..every line of it) before it goes to the client. Engineering? One engineer builds the spec, and another has to approve it; this is actually mandated by law for a lot of things, in fact, where permitting is involved (like construction).
Fundamentally, the question is "how to you keep code from being pushed to the public before it's tested." You seemed to miss that in your reply, because the very point of the question requires two people...people who must understand what their reading (and thus, are coders)...to look at the code. Also, your reply seems to imply that a code change requires reading ALL of the code, not just the new or changed code, and this is simply not true.