POSTing a new review request isn't supposed to attempt to create or

update from a commit ID unless create_from_commit_id is passed in the

request. We weren't checking this flag, though, before looking up an

existing review request with that commit ID. This triggered a

ChangeNumberInUseError, which ended up confusing RBTools.
We no longer make this check unless being told to create or update from

the commit ID.

Unit tests passed.
Posted a change twice. Got two review requests.