Re: in_merge_bases() is too expensive for recent "pu" update

> Well, yeah, you snipped this part from my original post :-)
>
> } Even if this turns out to be flawed, we should also identify uses of
> } in_merge_bases() where the real question was is_descendant_of() [I
> } somewhat suspect that's all of them], and then replace is_descendant_of
> } with a much cheaper lookup.  This can be as simple as propagating a mark
> } from the candidate until it either goes beyond all possible ancestors,
> } or hits one of them.