Add an array of parents to the `CommitHeader` type

[sebastinez]

We currently are missing a listing of the parent commits of a specific commit

export interface CommitHeader {
  author: Author;
  committer: Author;
  committerTime: number;
  description: string;
  sha1: string;
  summary: string;
}

We should add the following property to this interface

parents: string[]

This would also need a rewrite on the http-api to use radicle_surf instead of radicle_source as currently used.