The standard git workflow (and this github feature) seems to promote resolving the conflicts alongside all of the other changes in the merge working copy. This make me nervous, as there's no way to differentiate the new lines that were introduced to resolve merge conflicts from the thousands of lines of (previously reviewed) code from the feature branch.
If you're not careful, completely unrelated working-copy code and behavior can be introduced in a "merge commit" and neither you or any of your reviewers will notice. "Looks good to me."