Abstract some common code into a helper function.

* subversion/bindings/swig/python/libsvn_swig_py/swigutil_py.c
  (cstring_to_pystring): New helper function.
  (convert_svn_client_commit_item3_t): Use cstring_to_pystring() to
    avoid repetitious code.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1465046 13f79535-47bb-0310-9956-ffa450edef68