replace_nonprintable: Keep \n around

Since it has a functional role, we can not just replace it, we must keep
it around. This also allows us to simplify the code slightly.

We must fix this before we fix #1438 since otherwise the \n will be
missing with --style=plain, since we will stop adding it if it is
missing.

src/preprocessor.rs

@@ -72,7 +72,7 @@ pub fn replace_nonprintable(input: &[u8], tab_width: usize) -> String {
                     }
                 }
                 // line feed
-                '\x0A' => output.push('␊'),
+                '\x0A' => output.push_str("␊\x0A"),
                 // carriage return
                 '\x0D' => output.push('␍'),
                 // null

src/printer.rs

@@ -91,9 +91,6 @@ impl<'a> Printer for SimplePrinter<'a> {
             if self.config.show_nonprintable {
                 let line = replace_nonprintable(line_buffer, self.config.tab_width);
                 write!(handle, "{}", line)?;
-                if line_buffer.last() == Some(&b'\n') {
-                    writeln!(handle)?;
-                }
             } else {
                 handle.write_all(line_buffer)?
             };