Fix selections when opening excerpt with an existing buffer that has expanded diff hunks (#44360)

Release Notes:

- N/A

crates/editor/src/editor.rs

@@ -22102,14 +22102,23 @@ impl Editor {
                             None => Autoscroll::newest(),
                         };
                         let nav_history = editor.nav_history.take();
+                        let multibuffer_snapshot = editor.buffer().read(cx).snapshot(cx);
+                        let Some((&excerpt_id, _, buffer_snapshot)) =
+                            multibuffer_snapshot.as_singleton()
+                        else {
+                            return;
+                        };
                         editor.change_selections(
                             SelectionEffects::scroll(autoscroll),
                             window,
                             cx,
                             |s| {
                                 s.select_ranges(ranges.into_iter().map(|range| {
-                                    // we checked that the editor is a singleton editor so the offsets are valid
-                                    MultiBufferOffset(range.start.0)..MultiBufferOffset(range.end.0)
+                                    let range = buffer_snapshot.anchor_before(range.start)
+                                        ..buffer_snapshot.anchor_after(range.end);
+                                    multibuffer_snapshot
+                                        .anchor_range_in_excerpt(excerpt_id, range)
+                                        .unwrap()
                                 }));
                             },
                         );