Make 'svnadmin verify' dectect cycles in our DAG and prevent stack overflow
during that check.  Found by fuzzing tests.

* subversion/libsvn_fs_fs/tree.c
  (verify_node): Add list of parent nodes as extra parameter and check for
                 matches with the current node, i.e. cycles.
  (svn_fs_fs__verify_root): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1623398 13f79535-47bb-0310-9956-ffa450edef68