Optimize enumerating through svn_config_t options values.

* subversion/libsvn_subr/config.c
  (make_string_from_option): Attempt to expand option value only if '%' 
   character is present.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1464478 13f79535-47bb-0310-9956-ffa450edef68