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