See all open pull requests for a release:

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 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