api: fix get_changeset() when incomplete raw_id is passed with with_reviews

Previously, ChangesetStatusModel was queried with the raw_id passed as an
argument to the API function. When the raw_id was incomplete (i.e. shortened),
no reviews were found. Using the full raw_id from the changeset instance fixes
that.

Someone might argue that the caller is supposed to pass a full raw_id to the
API function. However, in any case, the return value should not be incomplete
without notice.