You can push up a series of commits, but each one is it's own code review so you want to do it less often. Another thing is if you have to add something to the HEAD^^ you have to rearrange your history and squash things together, but in the end the history is really clean. Also, each push of of each commit is versioned so you can easily tell what has changed between the last code review and the current; this is impossible with GitHub if you want to keep your history clean and compare different versions of the code review.