Do not lint write_with_newline when there is more than one newline

[dtolnay]

My code uses:

write!(file, "// THIS FILE IS AUTOMATICALLY GENERATED; DO NOT EDIT\n\n")?;

I believe the above is clearer than Clippy's suggestion, in which one newline is written by the macro and one newline is written by my string:

writeln!(file, "// THIS FILE IS AUTOMATICALLY GENERATED; DO NOT EDIT\n")?;

Mentioning @estk who worked on this lint in #2630.

[estk]

This makes sense to me, @oli-obk?

[ghost]

#2295 is similar.

[oli-obk]

@estk yea, let's not lint on multiple trailing newlines, only on a single one

[estk]

I think ill have time to knock this one out within a week.