Better detection of the directory which-error.py is in.

* tools/dev/which-error.py
  (get_errors): Use os.path.realpath(__file__) to allow the command to be
    symlinked to.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1547977 13f79535-47bb-0310-9956-ffa450edef68