Allow fetching a file via the API using the pull request ref. 

See this thread from drone: https://discourse.drone.io/t/new-commits-in-an-open-pull-request/868/6

Drone right now is fetching `.drone.yml` from main repo. It means that if there is an open pull request that changes drone config, it will still be fetched from main repo, not from pull request change.

Is this still unavailable? Help can be offered if pointed to the right direction.