feat: impl revwalk

[seokju-na]

Add revwalk feature.

import { openRepository } from 'es-git';

const repo = await openRepository('...');
const revwalk = repo.revwalk().pushHead();
console.log([...revwalk]); // ['a01e9888e46729ef4aa68953ba19b02a7a64eb82', 'b33e0101b828225f77eeff4dfa31259dcf379002']