Update `GitStore.on_buffer_store_event` so that, when a `BufferStoreEvent::BufferChangedFilePath` event is received, we check if there's any diff state for the buffer and, if so, update it according to the new file path, in case the file exists in the repository. Closes #40499 Release Notes: - Fixed issue with git diff tracking when updating a buffer's file from an untracked to a tracked file