On the fsfs-format7 branch: fix the way we detect that a given
container contains the wanted item.

* subversion/libsvn_fs_fs/cached_data.c
  (block_read): select by file offset; check correct sub-item

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1467395 13f79535-47bb-0310-9956-ffa450edef68