Keep a built leaf when the view lags the tree

A chat opens again after a crash between the build of a free leaf and
the save of the view. The catch-up adds the messages that the view does
not cover, and it now keeps a leaf that the tree holds already instead
of a second build of it, which the tree refuses as a duplicate. The
parent of such a leaf is settled at the start, with the other built
nodes.

A process that is killed during writes then opens again, so the log
loses no message that it acknowledged.

Issue: 5daf23e