msglist: Send positive numAfter for fetchInitial

This is effectively NFC given normal server behavior.  In particular,
the Zulip server is smart enough to skip doing any actual work to
fetch later messages when the anchor is already `newest`.

When we start passing anchors other than `newest`, we'll need this.

Author: gnprice

lib/model/message_list.dart

@@ -561,7 +561,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       narrow: narrow.apiEncode(),
       anchor: AnchorCode.newest,
       numBefore: kMessageListFetchBatchSize,
-      numAfter: 0,
+      numAfter: kMessageListFetchBatchSize,
       allowEmptyTopicName: true,
     );
     if (this.generation > generation) return;

test/model/message_list_test.dart

@@ -157,7 +157,7 @@ void main() {
           narrow: narrow.apiEncode(),
           anchor: 'newest',
           numBefore: kMessageListFetchBatchSize,
-          numAfter: 0,
+          numAfter: kMessageListFetchBatchSize,
           allowEmptyTopicName: true,
         );
       }

test/widgets/message_list_test.dart

@@ -404,7 +404,7 @@ void main() {
             'narrow': jsonEncode(narrow.apiEncode()),
             'anchor': AnchorCode.newest.toJson(),
             'num_before': kMessageListFetchBatchSize.toString(),
-            'num_after': '0',
+            'num_after': kMessageListFetchBatchSize.toString(),
             'allow_empty_topic_name': 'true',
           });
       });
@@ -437,7 +437,7 @@ void main() {
             'narrow': jsonEncode(narrow.apiEncode()),
             'anchor': AnchorCode.newest.toJson(),
             'num_before': kMessageListFetchBatchSize.toString(),
-            'num_after': '0',
+            'num_after': kMessageListFetchBatchSize.toString(),
             'allow_empty_topic_name': 'true',
           });
       });