control-chars.py: Minor refactoring.

Separate the "walk svn_repos_node_t tree" logic from the "visit a node" logic.

* tools/hook-scripts/control-chars.py
  (check_tree): Bifurcated into...
  (check_node, walk_tree): ... these two functions.
  (main): Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1461392 13f79535-47bb-0310-9956-ffa450edef68