hgweb: add a link to followlines in descending direction

We change the content of the followlines popup to display two links inviting
to follow the history of selected lines in ascending (as before) and
descending directions. The popup now renders as:

  follow history of lines <fromline>:<toline>:
  <a href=...>ascending</a> / <a href=...>descending</a>

mercurial/templates/static/followlines.js

@@ -204,13 +204,23 b" document.addEventListener('DOMContentLoa"
         //   <div class="followlines-link">
         var aDiv = document.createElement('div');
         aDiv.classList.add('followlines-link');
-
-        //     <a href="/log/<rev>/<file>?patch=&linerange=...">
-        var a = document.createElement('a');
+        aDiv.textContent = 'follow history of lines ' + fromline + ':' + toline + ':';
+        var linesep = document.createElement('br');
+        aDiv.appendChild(linesep);
+        //     link to "ascending" followlines
+        var aAsc = document.createElement('a');
         var url = targetUri + '?patch=&linerange=' + fromline + ':' + toline;
-        a.setAttribute('href', url);
-        a.textContent = 'follow lines ' + fromline + ':' + toline;
-        aDiv.appendChild(a);
+        aAsc.setAttribute('href', url);
+        aAsc.textContent = 'ascending';
+        aDiv.appendChild(aAsc);
+        var sep = document.createTextNode(' / ');
+        aDiv.appendChild(sep);
+        //     link to "descending" followlines
+        var aDesc = document.createElement('a');
+        aDesc.setAttribute('href', url + '&descend=');
+        aDesc.textContent = 'descending';
+        aDiv.appendChild(aDesc);
+
         div.appendChild(aDiv);
 
         return [div, button];