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