Add the ability to propose changes to a set of buffers (#18170)

This PR introduces functionality for creating *branches* of buffers that
can be used to preview and edit change sets that haven't yet been
applied to the buffers themselves.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>
Co-authored-by: Marshall <marshall@zed.dev>

crates/project/src/project_tests.rs

@@ -3288,7 +3288,7 @@ async fn test_buffer_is_dirty(cx: &mut gpui::TestAppContext) {
         cx.subscribe(&buffer1, {
             let events = events.clone();
             move |_, _, event, _| match event {
-                BufferEvent::Operation(_) => {}
+                BufferEvent::Operation { .. } => {}
                 _ => events.lock().push(event.clone()),
             }
         })