Exclude PRs merged but not in releasing branch #50

Is your feature request related to a problem? Please describe.

Not all merged PRs are merged into the currently to-be-released branch. It would be useful to include only those that are merged into the current branch.

Describe the solution you'd like

Specify a branch I am releasing from (default: current branch or repo default), and only include PRs that have been merged into that branch.

The simplest solution is to look at the base branch in the pr metadata. This, however, will not include multi-stage PRs (multiple PRs to a feature branch, which is later merged to main). It's a good start, though.

It is possible from the git graph, though, to determine exactly which PRs are ancestors to the current branch. That might be more work than we want to do, and can always be a second improvement later.

Additional context

I'm working on a jupyerhub release, and we currently have a long-running 'rbac' branch with merged PRs that are not being released. I have to manually remove those from the generated changelog.

In the future, though, I'd like all of those to be included as changelog items, rather than the one big rbac->main merge at the end.