always use new_index

my manual tests had always had an equal number of insertions and deletions, so I
didn't notice this. if you use old_index here then you end up with duplicate
line numbers for any pure insertion, such as adding a required import

Author: ntBre

crates/ruff_db/src/diagnostic/render/full.rs

@@ -151,7 +151,7 @@ impl std::fmt::Display for Diff<'_> {
                             "|",
                             self.stylesheet.none,
                             self.stylesheet.line_no,
-                            change.old_index(),
+                            change.new_index(),
                         ),
                     };