Minor speedup to node verification in FSFS (FSX will use a different
approach) by skipping full DAG node creation unless necessary.

* subversion/libsvn_fs_fs/tree.c
  (verify_node): get info directly from the noderev when feasible

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520421 13f79535-47bb-0310-9956-ffa450edef68