Generating release changelog from non-master branch commit history

We have a system where the latest major release is always from master branch, but we also maintain old major releases in branches such v1.x, v2.x etc, etc. If I run gren for a release from master everything works as I'd expect. The release notes for the latest major release are generated and include all the commits from master branch, but if I want to make a new oldversion.x.y release I can't work out how to make gren use the commits from the appropriate branch rather than master. I couldn't see anything in the docs: https://github-tools.github.io/github-release-notes/options.html

Is this possible and I've just overlooked the option I need?

Invoking gren like this FWIW:
```
$GREN \
--username=$USER \
--repo=$REPO \
--token=$GITHUB_TOKEN \
--data-source=commits \
--action=release \
--tags=$VERSION \
--include-messages=commits
```