Partial checksums

This commit adds a function to accept partial checksums.  I worked on it a few weeks ago but came back to it in context of our conversation about check-diff and cleaning up downloaded commits.  But because the code structure only deals with checksums, not accounting for their deploy serials, and skips repeated checksums, its functionality is unchanged by check-diff in rpm-ostree upgrade.  So I figured I would clean up the code and send in a pull request for feedback =)