Track dirtyness per item (#26237)

This reduces the number of multibuffer syncs when starting the editor
with 80
files open in the Zed repo from 10,000,000 to 100,000 by avoiding
O(n**2)
dirtyness checks.

Release Notes:

- Fixed a beachball when restarting in a large repo with a large number
open files

crates/workspace/src/item.rs

@@ -802,6 +802,7 @@ impl<T: Item> ItemHandle for Entity<T> {
                         }
 
                         ItemEvent::UpdateTab => {
+                            workspace.update_item_dirty_state(item, window, cx);
                             pane.update(cx, |_, cx| {
                                 cx.emit(pane::Event::ChangeItemTitle);
                                 cx.notify();