We made the flags that enable recursive merge internal, on the
assumption that nobody would want them and they're hard to reason about.
(Giving people an option that nobody wants is just extra noise.)

However, it made it hard for _us_ to reason about.  There's no good
reason to keep it private, let's just make it public and push that
cognitive load onto our poor users.  But they should expect it, they're
dealing with git, after all.