When the Review Board server has the review_request.supports_history

capability, we will default to posting with history. Using history can

be forced with the new -H/--with-history flag, which will cause an

error if the server does not support it.
Traditional-style squashed review requests are still supported: they can

be posted with -S/--squash-history.
Review requests that were created with history will always be updated

with history and review requests not created with history will,

likewise, never be updated with history. Since review requests in

Review Board exist in one state or the other, attempting to post would

result in an error, so we do not let users attempt such posts.
If the server does not support history, the -S and -H options are

ignored.

Posted this review request.

Posted a review request with multiple commits to my devserver.