Issue Type: Feature Request

Hi,

I'm working on supporting rich reviews in live share collaboration session. To support that, we'd need the following APIs:

1. Repository.getGlobalConfig(key: string): Promise<string>
2. Repository.getLog(options?: GitLogOptions): Promise<Commit>
3. Repository.diffWorkingDirectoryWith(ref: string): Promise<Change[]>
4. Add interface GitLogOptions { readonly maxEntries?: number; }
5. Add readonly authorEmail?: string | undefined; to Commit interface.

I have changes ready and can submit a PR if you accept this feature request.