https://github.com/pulls?q=is:pr+org:edx+base:open-release/koa.master+is:open
To see all the changes between start-ref and end-ref (including the end, but not including the start):
$ git log end-ref ^start-ref
For example, what is on koa.master since koa.test01?
$ git log origin/open-release/koa.master ^origin/open-release/koa.test01