title: Configure Hg to not ask for user name and password on command line
body: <p>When I try to clone a https-authenticated Mercurial repository (e.g. from kiln), it asks me for my user name</p>
  
  <pre><code>C:\temp\test&gt;hg clone https://&lt;my-login-name&gt;.kilnhg.com/Code/Repositories/Group/test
  http authorization required
  realm: kiln
  user:
  </code></pre>
  
  <p>We want to invoke the <code>hg clone</code> command from our GUI application with any URL provided by the user and ask the user <em>on demand</em> for the user name and password, if they are required.</p>
  
  <p>How can I configure Mercurial to invoke an arbitrary script which returns the user name/password, similar to scripts whose paths are defined in the environment variables <code>GIT_ASKPASS</code> or <code>SSH_ASKPASS</code> for Git?</p>