Restore project diff test (#21606)

Restores a basic project diff test

Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/project/src/buffer_store.rs

@@ -49,6 +49,7 @@ struct SharedBuffer {
     unstaged_changes: Option<Model<BufferChangeSet>>,
 }
 
+#[derive(Debug)]
 pub struct BufferChangeSet {
     pub buffer_id: BufferId,
     pub base_text: Option<Model<Buffer>>,
@@ -1045,6 +1046,12 @@ impl BufferStore {
             .spawn(async move { task.await.map_err(|e| anyhow!("{e}")) })
     }
 
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn set_change_set(&mut self, buffer_id: BufferId, change_set: Model<BufferChangeSet>) {
+        self.loading_change_sets
+            .insert(buffer_id, Task::ready(Ok(change_set)).shared());
+    }
+
     pub async fn open_unstaged_changes_internal(
         this: WeakModel<Self>,
         text: Result<Option<String>>,