Fix slow loading of large summaries.

- e.g 40 secs -> 0.5 secs
- The whole summary was being scanned for every entry added.
  quadratic -> linear.

eris/__main__.py

@@ -281,7 +281,8 @@ class Summary:
         if entry_index <= y:
             self.scroll(0, -1)
         self._jobs_added_event.set()
-        self.closest_placeholder_generator = None
+        if self.is_loaded:
+            self.closest_placeholder_generator = None
 
     def on_file_added(self, path):
         full_path = os.path.join(self._root_path, path)