4.6.2 Disable fork mode with `fork=false`

Pull request: #141

Adds the ability to create backports in the origin repository instead of in a fork. Fork mode is still the default. To disable fork mode run backport --fork=false or add it to the project config:

{
  "upstream": "elastic/kibana",
  "branches": [{ "name": "7.x", "checked": true }, "7.3", "7.2"],
  "fork": false
}