In the ra serf (update) editor, spool the report to a spillbuffer instead of
to a tempfile to avoid creating a tempfile for small requests.

* subversion/libsvn_ra_serf/update.c
  (report_context_t): Update variable type. Remove TODO.
  (set_path,
   delete_path,
   link_path): Update caller.
  (create_update_report_body): Decide whether to send a file or a buffer.
    Move sendfile optimization here.
  (finish_report): Update caller. Move sendfile optimization to
    create_update_report_body.
  (make_update_reporter): Create spillbuf instead of tempfile. Update caller.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1555716 13f79535-47bb-0310-9956-ffa450edef68