extensions/git/src/util.ts/groupBy
export function groupBy<T>(arr: T[], fn: (el: T) => string): { [key: string]: T[] } {
	return arr.reduce((result, el) => {
		const key = fn(el);
		result[key] = [...(result[key] || []), el];
		return result;
	}, Object.create(null));
}