Fix routing of leader updates from unshared projects

Previously, leader updates in unshared projects would be sent to
all followers regardless of project, as if they were not scoped
to any project.

crates/editor/src/items.rs

@@ -82,7 +82,9 @@ impl FollowableItem for Editor {
 
         let pane = pane.downgrade();
         Some(cx.spawn(|mut cx| async move {
-            let mut buffers = futures::future::try_join_all(buffers).await?;
+            let mut buffers = futures::future::try_join_all(buffers)
+                .await
+                .debug_assert_ok("leaders don't share views for unshared buffers")?;
             let editor = pane.update(&mut cx, |pane, cx| {
                 let mut editors = pane.items_of_type::<Self>();
                 editors.find(|editor| {