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 .

Start at the `gren.js release --data-source=commits` entry point and inspect how the date range and master branch determine the commit history. Done means a starting commit SHA can be provided for release-branch workflows and release notes are generated from the intended commits; verify the behavior with the relevant release-notes tests.

索引モデルが issue の本文から書いたものです。