[MM-16667] Migrate User.GetByUsername to Sync by default #11453 (#11520)

* Migrate User.GetByUsername to Sync by default #11453

* fix app errors #11453

* fix app errors #11453

* fix cmd error #11453

* fix shadow var #11453

* fix shadow user var #11453

* fix test error

* fix test error #11453

cmd/mattermost/commands/userargs.go

@@ -21,8 +21,9 @@ func getUserFromUserArg(a *app.App, userArg string) *model.User {
 	user, _ := a.Srv.Store.User().GetByEmail(userArg)
 
 	if user == nil {
-		if result := <-a.Srv.Store.User().GetByUsername(userArg); result.Err == nil {
-			user = result.Data.(*model.User)
+		var err *model.AppError
+		if user, err = a.Srv.Store.User().GetByUsername(userArg); err == nil {
+			return user
 		}
 	}