New excerpt controls (#24428)

Release Notes:

- Multibuffers now use less vertical space for excerpt boundaries.
Additionally the expand up/down arrows are hidden at the start and end
of the buffers

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Zed AI <claude-3.5-sonnet@zed.dev>

crates/collab/src/tests/following_tests.rs

@@ -313,9 +313,8 @@ async fn test_basic_following(
         result
     });
     let multibuffer_editor_a = workspace_a.update_in(cx_a, |workspace, window, cx| {
-        let editor = cx.new(|cx| {
-            Editor::for_multibuffer(multibuffer_a, Some(project_a.clone()), true, window, cx)
-        });
+        let editor = cx
+            .new(|cx| Editor::for_multibuffer(multibuffer_a, Some(project_a.clone()), window, cx));
         workspace.add_item_to_active_pane(Box::new(editor.clone()), None, true, window, cx);
         editor
     });