Clear selection when setting completion pattern.

It doesn't make sense to have an active selection while you are
filtering by entering text. You should be in one of two states:

1. Tabbing through completions (valid selection)
2. Entering a filter pattern (invalid selection)

Fixes #2843, where a crash would occur after the following:

1. tab to an item other than the first
2. <backspace>
3. re-type last character
4. <ctrl-d>

This would try to delete an out of range index.

qutebrowser/completion/completionwidget.py

@@ -297,6 +297,7 @@ class CompletionView(QTreeView):
         self.pattern = pattern
         with debug.log_time(log.completion, 'Set pattern {}'.format(pattern)):
             self.model().set_pattern(pattern)
+            self.selectionModel().clear()
             self._maybe_update_geometry()
             self._maybe_show()
 

tests/unit/completion/test_completionwidget.py

@@ -87,6 +87,7 @@ def test_set_pattern(completionview):
     completionview.set_model(model)
     completionview.set_pattern('foo')
     model.set_pattern.assert_called_with('foo')
+    assert not completionview.selectionModel().currentIndex().isValid()
 
 
 def test_set_pattern_no_model(completionview):