message view: Do not mark selected message as read on narrow.

For narrows where we select the first unread message
on activation, we also `force_rerender` the message
list which causes the message to get reselected.

Thus when the `MessageList.select_id` is called for
the first time, `previously_selected` is -1. However,
due to this rerender on selecting the message again
the previously selected message and our to be selected
message is the same.

On passing this information to the `message_selected.zulip`
event, the event handler in `message_scroll.js` tries to
mark the message as read. The message is marked on the
second event instead of the first as the previously
selected message is -1.

Fixes zulip#10886.

static/js/message_scroll.js

@@ -166,7 +166,7 @@ exports.initialize = function () {
 
     // Scroll handler that marks messages as read when you scroll past them.
     $(document).on("message_selected.zulip", (event) => {
-        if (event.id === -1) {
+        if (event.id === -1 || event.id === event.previously_selected) {
             return;
         }