Add orphan/force push option

[MoOx]

When you have a website with a lot of pages, versionning gh-pages can start to be a problem. I have a repo that is almost 100MB for the single gh-pages. I would like to add an option to checkout --orphan gh-pages and so be able to force push. Are you open to a PR for this option (name: orphanBranch) ?

[tschaub]

There is an undocumented depth option. I'm curious if setting depth: 1 would help your case.

If so, I'd be interested in making it the default and adding a shallow option. If true (the default), we could git clone --depth 1. Otherwise, git clone without the --depth arg.

[MoOx]

It's not what I want. I want to have a single commit in gh-pages all the time, not just while I am cloning.

I think for my need I can do with a simple command line like this https://github.com/facebook/graphql/blob/e27c004cbc7f752b5ea816375215fe333e02c9e/package.json#L21