Use new multibuffer excerpts in find-all-references and friends (#27876)

Release Notes:

- N/A

---------

Co-authored-by: Kirill Bulatov <mail4score@gmail.com>

crates/editor/src/editor_tests.rs

@@ -15824,7 +15824,7 @@ async fn test_display_diff_hunks(cx: &mut TestAppContext) {
         for buffer in &buffers {
             let snapshot = buffer.read(cx).snapshot();
             multibuffer.set_excerpts_for_path(
-                PathKey::namespaced("", buffer.read(cx).file().unwrap().path().clone()),
+                PathKey::namespaced(0, buffer.read(cx).file().unwrap().path().clone()),
                 buffer.clone(),
                 vec![text::Anchor::MIN.to_point(&snapshot)..text::Anchor::MAX.to_point(&snapshot)],
                 DEFAULT_MULTIBUFFER_CONTEXT,