Four rows go to yes for iOS: draft/ready, retarget, request a review,
and stacked merge requests.
krz/gitbay-ios!50 decodes draft, review_requests, stacked_on and
stacked — all already on the wire — and dispatches mr draft,
mr ready, mr retarget and mr review request. The merge control
hides squash and rebase while a stack exists, matching what the server
refuses, and keeps both fast-forward and a merge commit, matching what it
permits.