Fix a bug in tree building
Fix a bug in tree building

Concurrent subtrees builds did not discover the same subtree nodes as that was
only registered with the parent node asynchronously. This led to some subtrees
going missing.

cscott/restbase,wikimedia/mediawiki-services-restbase,wikimedia/mediawiki-services-restbase,eevans/restbase,Krenair/restbase,gwicke/restbase,Krenair/restbase,physikerwelt/restbase,wikimedia/hyperswitch,d00rman/restbase,milimetric/restbase,eevans/restbase,wikimedia/restbase,milimetric/restbase,inverno/restbase,Pchelolo/restbase,cscott/restbase,d00rman/restbase,Pchelolo/restbase,gwicke/restbase,wikimedia/restbase,inverno/restbase,physikerwelt/restbase