Minor tuning of our all-time favorite hot path: get_dag.

* subversion/libsvn_fs_fs/tree.c
  (get_dag): unconditionally canonicalize the path - see comment


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1533396 13f79535-47bb-0310-9956-ffa450edef68