Rustfmt sometimes puts 2 args in one line, but the rest on separate lines

[Boscop]

rustfmt 1.4.18-nightly (c1e9b7b 2020-06-13)

When I run rustfmt on this, it stays like this:

	self.renderer.render(
		texture,
		identity, // model
		identity, // perspective
		identity, // view
		None,     // alpha
		false,    // depth_test
		&mut target,
	);

But when I remove &mut before target:

	self.renderer.render(
		texture,
		identity, // model
		identity, // perspective
		identity, // view
		None,     // alpha
		false,    // depth_test
		target,
	);

rustfmt formats it like this:

	self.renderer.render(
		texture, identity, // model
		identity, // perspective
		identity, // view
		None,     // alpha
		false,    // depth_test
		target,
	);

Note: texture, identity, // model in one line.
:(

Expected output

	self.renderer.render(
		texture,
		identity, // model
		identity, // perspective
		identity, // view
		None,     // alpha
		false,    // depth_test
		target,
	);

[ytmimi]

@calebcartwright this is the same issue that we're dealing with in #5337 (comment), and I think we'll need a new variant to resolve it.