When the root set contains only one empty container with one child
first promote the child container to the root set and only then
remove the empty parent container so that the root set never goes
empty.
Also make mu_container_splice_children() do only one thing, that is
promote one container's children to be another container's siblings.
The resultant childless container is no longer removed by this
function.
Fixes#460.
The problem was that once a container got a parent, it did not change it anymore
due to the child_elligible condition, but the parent might have been assigned
from an incomplete References sequence.
Now, we make sure the last reference gets to be the message's parent (following
the JWZ's algorithm), reparenting the message if necessary. This makes sense, as
the last parent-child relationship (between last ref and the message) is the
most reliable piece of info here.
Instead of child_elligible, we now only check that the new parent is not a
descendant of the current message, to prevent making a loop. Everything else is
fine, as it only moves a subtree around.
