Add first snapshot tests using insta

[cruessler]

This is a follow-up to #2411, with an important fix. The original PR was reverted in 6d62241 due to a regression in the stashes tab. When the app would open in the stashes tab, it would take a couple of seconds for the app to actually show content. The reason was that I had applied what looked like an optimization to me, but what actually apparently prevented an initial QueueEvent::Notify from being sent. This missing event caused the stashes view to only call self.draw(terminal)? on the next QueueEvent::Tick which by default is after 5 seconds (if you change TICK_INTERVAL to a different value, you’ll see that it determines how long the app stays empty). This fix restores the code that was initially taken from run_app in main.rs.

As far as I can tell, this could also be fixed by adding an initial self.draw(terminal)? call before the loop in run_main_loop, right after self.app.update()?.

// In `Gitui::run_main_loop`.
// This was the version that was part of #2411.
let event = select_event(
	&self.rx_input,
	&self.rx_git,
	&self.rx_app,
	&self.rx_ticker,
	&self.rx_watcher,
	&spinner_ticker,
)?;
// This was the version before #2411.
let event = if first_update {
	first_update = false;
	QueueEvent::Notify
} else {
	select_event(
		&self.rx_input,
		&self.rx_git,
		&self.rx_app,
		&self.rx_ticker,
		&self.rx_watcher,
		&spinner_ticker,
	)?
};