msglist [nfc]: Add public renarrowAndFetch method, and use internally

We'll use this in keyword search, coming up, for renarrowing when
the user submits a new keyword.

Author: chrisbobbe

lib/model/message_list.dart

@@ -618,6 +618,14 @@ class MessageListView with ChangeNotifier, _MessageSequence {
   Narrow get narrow => _narrow;
   Narrow _narrow;
 
+  /// Set [narrow] to [newNarrow], reset, [notifyListeners], and [fetchInitial].
+  void renarrowAndFetch(Narrow newNarrow) {
+    _narrow = newNarrow;
+    _reset();
+    notifyListeners();
+    fetchInitial();
+  }
+
   /// The anchor point this message list starts from in the message history.
   ///
   /// This is passed to the server in the get-messages request
@@ -1101,10 +1109,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     switch (propagateMode) {
       case PropagateMode.changeAll:
       case PropagateMode.changeLater:
-        _narrow = newNarrow;
-        _reset();
-        notifyListeners();
-        fetchInitial();
+        renarrowAndFetch(newNarrow);
       case PropagateMode.changeOne:
     }
   }