Factor out some common code in the ra_svn marshaller.

* subversion/libsvn_ra_svn/marshal.c
  (svn_ra_svn__write_ncstring): new internal function with most code taken
                                from svn_ra_svn__write_cstring
  (svn_ra_svn__write_string,
   svn_ra_svn__write_cstring): call the common function


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1486000 13f79535-47bb-0310-9956-ffa450edef68