Support fetching more than 1k changes post 2.8

The update to support the offset rather than sortkey based batching
neglected to account for a query that would need more than 2 batches.

Change-Id: I04ef3243b1fb20cd688af3227fd9998e850fdf9a

gertty/sync.py

@@ -354,6 +354,7 @@ class SyncProjectTask(Task):
         changes = []
         sortkey = ''
         done = False
+        offset = 0
         while not done:
             query = '&'.join(queries)
             # We don't actually want to limit to 500, but that's the server-side default, and
@@ -371,7 +372,8 @@ class SyncProjectTask(Task):
                     if '_sortkey' in batch[-1]:
                         sortkey = '&N=%s' % (batch[-1]['_sortkey'],)
                     else:
-                        sortkey = '&start=%s' % (len(batch))
+                        offset += len(batch)
+                        sortkey = '&start=%s' % (offset,)
         change_ids = [c['id'] for c in changes]
         with app.db.getSession() as session:
             # Winnow the list of IDs to only the ones in the local DB.