Fix signed/unsigned warnings in FSFS stats code.

* subversion/libsvn_fs_fs/stats.c
  (add_change): Change type of REP_SIZE and EXPANDED_SIZE arguments to 
   apr_uint64_t.
  (read_noderev): Do not cast apr_uint64_t to apr_int64_t when calling
   add_change().


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1647540 13f79535-47bb-0310-9956-ffa450edef68