bugfix: forgot to mark split containers as split = true (+test) (Thanks szalik)

When the workspace layout (formerly orientation) was forced to change
due to a move command, the split container we created was not marked as
split = true, which caused tree_flatten() to errnously kill the contents
of it and thus one window ended up unmanaged.

Also, the logic in tree_flatten() was inverted due to commit de94f6da.

fixes #790

src/tree.c

@@ -617,8 +617,8 @@ void tree_flatten(Con *con) {
 
     /* The child must have a different orientation than the con but the same as
      * the con’s parent to be redundant */
-    if (con->split ||
-        child->split ||
+    if (!con->split ||
+        !child->split ||
         con_orientation(con) == con_orientation(child) ||
         con_orientation(child) != con_orientation(parent))
         goto recurse;