Fix issue 4350, long keyword expansion.

* subversion/libsvn_subr/subst.c
  (translate_keyword_subst): Don't expand overlong keywords.

* subversion/tests/libsvn_subr/subst_translate-test.c
  (test_funcs): Mark test_svn_subst_long_keywords PASS.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467237 13f79535-47bb-0310-9956-ffa450edef68