title: Have Mercurial repo reject any push that would result in a multiheaded branch
body: <p>How can I configure a Mercurial repo to reject any push that would result in a multiheaded branch?</p>
  
  <p>As a follow up, how can I configure a clone to always pull before a commit, so hg can warn if the commit will result in a multiheaded branch, so the user can either abort or know that they must merge / rebase after the commit?</p>