In FSFS, revise the calls to get_dag that requested a locked node.
In several cases, the lock is not required.  Update those calls.

* subversion/libsvn_fs_fs/tree.c
  (fs_node_created_path): Copy the result requested, so we don't need the
                          node to remain valid.
  (get_root): Duplicate the cached node.  This function will be used at
              the beginning of e.g. commit().  So, it is not performance
              critical and we shouldn't lock the cache for the whole time.
  (crawl_directory_dag_for_mergeinfo,
   add_descendant_mergeinfo): It turns out that we never use two DAG nodes
                              at a time, so we don't need to lock them.
                              Document this and Update the get_dag call. 

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648241 13f79535-47bb-0310-9956-ffa450edef68