title: Get git SHA1 hash of a changeset (or diff) in a commit
body: <p>I'm mostly integrating finished topic branches into the main development branch, in git. While doing that I sometimes have to change the message of commit (improving, correcting spelling, ...) which results into a new commit with new SHA1 ID. </p>
  
  <p>I would like to get the <strong>SHA1 hash</strong> of the <strong>changes in a commit</strong>. This would let us verify if any content of the changeset is altered or not after modyfying the commit message.</p>
  
  <p>It would be great if we could get the SHA1 hash of the changeset of a <strong>sequence of commits</strong> too. Then we would be able to do interactive rebases where we squash commits and still end up with the same SHA1 hash of the changeset.</p>