Don't attempt to post changenums when posting a commit_id fails.

Due to a bug in the Review Board 2.0 betas, posting a new review request
with a commit ID matching an existing commit ID would cause us to
receive an error saying the change number was in use. We'd then try to
post again using the changenum instead of commit_id, which would fail
due to being None.

Part of this needs to be fixed in Review Board (as we shouldn't be
getting this error unless we're trying to create from that commit ID),
but we can also handle this better by falling back to the more general
error case.