Fix issue 4479, multiline svn:special truncated.

* subversion/libsvn_subr/subst.c
  (create_special_file_from_stream): Write out the whole file.

* subversion/tests/cmdline/special_tests.py
  (multiline_special): Remove XFAIL.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1577149 13f79535-47bb-0310-9956-ffa450edef68