Follow-up to r1648537: Make sure all callers of dag_node_cache_get
ultimately return nodes allocated in their respective result pools.

* subversion/libsvn_fs_fs/tree.c
  (get_dag): Auto-copy the DAG node before returning it.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648538 13f79535-47bb-0310-9956-ffa450edef68