R1:c596c358db11

Make "Reply" on ghost inlines work correctly (or, at least, consistently)

Summary:
Ref T7447. Ref T7870. When you "reply" to a ghost inline, make it work properly.

This exact behavior is arguable. In particular, when you reply to a ghost inline, we //could// put the reply on the same diff as the original.

I suspect it aligns better with user exepectation to put the new inline on the current (visible) diff instead, and generally for inlines to flow forward through time and all of the ghosts to pretty…