Fix some completion bugs

TODO

@@ -4,7 +4,6 @@ New config TODO
 - Changing models is awfully slow
 - Add correctly working :get and :set commands
 - Implement different layers of config options correctly, not only default.
-- When cmd string ends with space, the next model should be shown
 
 Bugs
 ====

qutebrowser/widgets/completion.py

@@ -205,7 +205,6 @@ class CompletionView(QTreeView):
 
         """
         # FIXME we should also consider the cursor position
-        # FIXME when backspacing, models aren't changed correctly
         if self._ignore_next:
             # Text changed by a completion, so we don't have to complete again.
             self._ignore_next = False
@@ -221,6 +220,8 @@ class CompletionView(QTreeView):
 
         text = text.lstrip(':')
         parts = split_cmdline(text)
+        if text.endswith(' '):
+            parts.append('')
 
         model = self._get_new_completion(parts)
         if model != self._lastmodel: