Add commit flag at the end of running migrate

[michaeljguarino]

Summary

It's very likely a large number of users will forget to manage their git, we should just remove that possibility w/ this.

Test Plan

n/a

Checklist

• [ ] If required, I have updated the Plural documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] I have added a meaningful title and summary to convey the impact of this PR to a user.
• [ ] I have added relevant labels to this PR to help with categorization for release notes.