hgweb: add appendFormatHTML javascript function

This is a convenient helper function to append some formatted HTML markup to a
DOM element.

mercurial/templates/static/mercurial.js

@@ -341,3 +341,7 b' function docFromHTML(html) {'
     doc.documentElement.innerHTML = html;
     return doc;
 }
+
+function appendFormatHTML(element, formatStr, replacements) {
+    element.insertAdjacentHTML('beforeend', format(formatStr, replacements));
+}