cmd/github-pull-request-make: use new API to fetch diffs
cmd/github-pull-request-make: use new API to fetch diffs

This means the tooling will work if we ever use it for private repos.

paperstreet/cockroach,petermattis/cockroach,glycerine/cockroach,glycerine/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,maxlang/cockroach,maxlang/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,paperstreet/cockroach,maxlang/cockroach,paperstreet/cockroach,petermattis/cockroach,maxlang/cockroach,maxlang/cockroach,maxlang/cockroach,petermattis/cockroach,petermattis/cockroach,petermattis/cockroach,vaibhavarora/cockroach,glycerine/cockroach,petermattis/cockroach,petermattis/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,maxlang/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,glycerine/cockroach,glycerine/cockroach