Introduce svn_stringbuf_from_stream() as symmetric counterpart to
svn_stream_from_stringbuf() and use it in one place.

* subversion/include/svn_io.h
  (svn_stringbuf_from_stream): declare new public API function

* subversion/libsvn_subr/stream.c
  (svn_stringbuf_from_stream): implement new API function

* subversion/libsvn_fs_fs/cached_data.c
  (get_dir_contents): use the new API

* develop/trunk/subversion/tests/libsvn_subr/stream-test.c
  (test_stringbuf_from_stream): new test case for new API
  (test_funcs): register new test case


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1532193 13f79535-47bb-0310-9956-ffa450edef68