manifest: remove dependency on treeinmem from manifest.add Currently manifest.add uses the treeinmem option to know if it can call fastdelta on the given manifest instance. In a future patch we will be moving add() to be on the manifestrevlog, so it won't have access to the treeinmem option anymore. Instead, let's have it actually check if the given manifest instance supports the fastdelta operation. This also means that if treemanifest or any implementation eventually implements fastdelta(), it will automatically benefit from this code path.