Be sure to skip the last backlog update

The instantaneous backlog processing introduced in commit 02a98fe
didn't skip the last backlog update, which wasn't the past and
correct behavior. This commit fixes this regression.

Regression-of: 02a98fe

Author: Pietro Albini

botogram/updates.py

@@ -54,11 +54,17 @@ def fetch(self, timeout=1):
         """Fetch the latest updates"""
         if not self._backlog_processed:
             # Just erase all the previous messages
-            self._bot.api.call("getUpdates", {
+            last = self._bot.api.call("getUpdates", {
                 "offset": -1,
                 "timeout": 0,
             }, expect=objects.Updates)
 
+            # Be sure to skip also the last update
+            if last:
+                self._last_id = last[-1].update_id
+            else:
+                self._last_id = 0
+
             self._backlog_processed = True
 
         updates = self._fetch_updates(timeout)