On the fsfs-format7 branch: fix the size calculation for plain reps.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__get_representation_length): negation was a leftover of the
                                          flag-based code

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1490961 13f79535-47bb-0310-9956-ffa450edef68