Follow-up to r1543623: Do the same to the noderevs container.

* subversion/libsvn_fs_x/noderevs.c
  (svn_fs_x__write_noderevs_container,
   svn_fs_x__read_noderevs_container): (de-)serialize revnums as signed ints

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1543629 13f79535-47bb-0310-9956-ffa450edef68