In FSFS, make get_dag not lock the DAG node in cache unless requested.
The critical part is the call to open_path which would always lock the
node even if we only want to have a single node returned.

* subversion/libsvn_fs_fs/tree.c
  (open_path_flags_t): Define a new flag.
  (open_path): Don't lock the DAG node returned if return only the last
               step in the path and don't explicitly ask for a lock.
               The internal iteration along the path is fine with only
               the lastest node being valid, i.e. at no point do we need
               to access two DAG nodes.
  (get_dag): Request a locked node only if we are told to do so.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1648238 13f79535-47bb-0310-9956-ffa450edef68