diffCallback = (err, diff) => {
					if (err) {
						oldAssets = undefined;
						callback(err);
						return;
					}
					applyDiff(
						fs,
						outputPath,
						dry,
						logger,
						/** @type {Diff} */ (diff),
						isKept,
						(err, keptAssets) => {
							if (err) {
								oldAssets = undefined;
							} else {
								if (oldAssets) mergeAssets(currentAssets, oldAssets);
								oldAssets = currentAssets;
								if (keptAssets) mergeAssets(oldAssets, keptAssets);
							}
							callback(err);
						}
					);
				}
@param {(Error | null)=} err err
@param {Diff=} diff diff