Avoid an unnecessary load_old_messages call on page load.

Previously, our check for whether we needed to call load_old_messages
a second time on page load to get up to the present caused us to
basically always do such a call.

(imported from commit b599041e8c0853b4c8c9ab2def6679142302523e)

zephyr/static/js/zephyr.js

@@ -907,8 +907,9 @@ $(function () {
         }
 
         // catch the user up
-        if (messages.length !== 1) {
+        if (messages.length !== 0) {
             var latest_id = messages[messages.length-1].id;
+            if (latest_id < page_params.max_message_id) {
                 load_old_messages({
                     anchor: latest_id,
                     num_before: 0,
@@ -918,6 +919,7 @@ $(function () {
                 });
                 return;
             }
+        }
         // now start subscribing to updates
         get_updates();