message_fetch: Pass msg_list to load_messages_for_narrow.

This tweak makes the code easier to reason about and may also help
generalize the codebase to support maintaining multiple message lists.

frontend_tests/node_tests/narrow_activate.js

@@ -169,10 +169,12 @@ run_test("basics", () => {
     let cont;
 
     message_fetch.load_messages_for_narrow = (opts) => {
+        // Only validates the anchor and set of fields
         cont = opts.cont;
 
         assert.deepEqual(opts, {
             cont: opts.cont,
+            msg_list: opts.msg_list,
             anchor: 1000,
         });
     };

static/js/message_fetch.js

@@ -292,13 +292,11 @@ export function load_messages(opts, attempt = 1) {
 }
 
 export function load_messages_for_narrow(opts) {
-    const msg_list = message_list.narrowed;
-
     load_messages({
         anchor: opts.anchor,
         num_before: consts.narrow_before,
         num_after: consts.narrow_after,
-        msg_list,
+        msg_list: opts.msg_list,
         cont: opts.cont,
     });
 }

static/js/narrow.js

@@ -534,6 +534,7 @@ export function activate(raw_operators, opts) {
                 msg_list.network_time = new Date();
                 maybe_report_narrow_time(msg_list);
             },
+            msg_list,
         });
     }