hgweb: insert followlines buttons before any children, including text nodes

This way the buttons come before any other content, including text nodes.
Because highlight extension replaces every line of text with some <span>
elements that have CSS classes for highlighting, the placement of followlines
buttons used to depend on if that extension was enabled or not. Let's make the
placement more consistent, it'll help the next patch in this series.

mercurial/templates/static/followlines.js

@@ -76,7 +76,7 b" document.addEventListener('DOMContentLoa"
         if ( childSupportElms.length > 0 ) {
             btnSupportElm = childSupportElms[0];
         }
-        var refNode = btnSupportElm.children[0]; // node to insert <button> before
+        var refNode = btnSupportElm.childNodes[0]; // node to insert <button> before
         btnSupportElm.insertBefore(btn, refNode);
     }