Provide commit sha to start with

Thanks for such a useful tool.

Current high level logic of `gren.js release --data-source=commits` is:
- determine a date range
- pick commits in master branch within this range
- create release notes using the commits

As a result, the following workflow is problematic (release notes are empty):
- create a new release branch from master
- merge to the release branch new changes
- tag the release branch
- generate release notes from the tag
- merge release to master

One option to fix it is to provide `sha` to start history with. Currently it is not provided so default master is used.

I will create a PR doing that. Also any workaround is appreciated.

The issue may be related to https://github.com/github-tools/github-release-notes/issues/195 and https://github.com/github-tools/github-release-notes/issues/116 .