Prepare the FSFS index creation code for upcomming changes.

* subversion/libsvn_fs_fs/index.c
  (stream_write_encoded): New utility function.
  (svn_fs_fs__l2p_index_append,
   svn_fs_fs__p2l_index_append): Write into a stream instead a plain file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631563 13f79535-47bb-0310-9956-ffa450edef68