function extract (dest) {
	const {pkgJson, target} = this
	const tmpDest = pkgJson.dist.path
	const where = path.join(dest, target, 'package')
	return util.rename(tmpDest, where)
		::retryWhen((errors) => errors::mergeMap((error) => {
			if (error.code !== 'ENOENT') {
				throw error
			}
			return util.mkdirp(path.dirname(dest))
		}))
}
extract a cloned git repository to destination.
@param {String} dest - pathname into which the cloned repository should be
extracted.
@return {Observable} - observable sequence that will be completed once
the cloned repository has been extracted.