Fix flicker when showing backround fetch status (#4223)

- **PR Description**

Fix occasional ugly flicker when showing the background fetch status
(recently introduced with #4215).

- **Please check if the PR fulfills these requirements**

* [x] Cheatsheets are up-to-date (run `go generate ./...`)
* [x] Code has been formatted (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#code-formatting))
* [ ] Tests have been added/updated (see
[here](https://github.com/jesseduffield/lazygit/blob/master/pkg/integration/README.md)
for the integration test guide)
* [ ] Text is internationalised (see
[here](https://github.com/jesseduffield/lazygit/blob/master/CONTRIBUTING.md#internationalisation))
* [ ] If a new UserConfig entry was added, make sure it can be
hot-reloaded (see
[here](https://github.com/jesseduffield/lazygit/blob/master/docs/dev/Codebase_Guide.md#using-userconfig))
* [ ] Docs have been updated if necessary
* [x] You've read through your own file changes for silly mistakes etc

Author: stefanhaller

pkg/gui/background.go

@@ -76,9 +76,9 @@ func (self *BackgroundRoutineMgr) startBackgroundFetch() {
 	self.gui.waitForIntro.Wait()
 
 	fetch := func() error {
-		return self.gui.PopupHandler.WithWaitingStatusSync(self.gui.Tr.FetchingStatus, func() error {
+		return self.gui.helpers.AppStatus.WithWaitingStatusImpl(self.gui.Tr.FetchingStatus, func(gocui.Task) error {
 			return self.backgroundFetch()
-		})
+		}, nil)
 	}
 
 	// We want an immediate fetch at startup, and since goEvery starts by

pkg/gui/controllers/helpers/app_status_helper.go

@@ -60,9 +60,13 @@ func (self appStatusHelperTask) Continue() {
 // withWaitingStatus wraps a function and shows a waiting status while the function is still executing
 func (self *AppStatusHelper) WithWaitingStatus(message string, f func(gocui.Task) error) {
 	self.c.OnWorker(func(task gocui.Task) error {
-		return self.statusMgr().WithWaitingStatus(message, self.renderAppStatus, func(waitingStatusHandle *status.WaitingStatusHandle) error {
-			return f(appStatusHelperTask{task, waitingStatusHandle})
-		})
+		return self.WithWaitingStatusImpl(message, f, task)
+	})
+}
+
+func (self *AppStatusHelper) WithWaitingStatusImpl(message string, f func(gocui.Task) error, task gocui.Task) error {
+	return self.statusMgr().WithWaitingStatus(message, self.renderAppStatus, func(waitingStatusHandle *status.WaitingStatusHandle) error {
+		return f(appStatusHelperTask{task, waitingStatusHandle})
 	})
 }