When posting a change for review with a commit ID and then discarding

it, that discarded review request still retains the commit ID. This

results in later conflicts with newly posted review requests, which

don't appear to break until the draft is published.
We now always remove the commit ID from the review request in order to

prevent any conflicts. While this is removing some history, it's history

that causes more harm than good and, since the review request is

discarded, it's not likely that the commit ID association is needed.

Unit tests pass.