Restore file to original content when rejecting file recreated by agent (#29264)

Release Notes:

- Fixed a bug that could sometimes cause a file to be deleted when
rejecting an agent change.

crates/agent/src/agent_diff.rs

@@ -988,7 +988,7 @@ mod tests {
             .await
             .unwrap();
         cx.update(|_, cx| {
-            action_log.update(cx, |log, cx| log.buffer_read(buffer.clone(), cx));
+            action_log.update(cx, |log, cx| log.track_buffer(buffer.clone(), cx));
             buffer.update(cx, |buffer, cx| {
                 buffer
                     .edit(