Fix the rev / pack file re-open logic after a background pack run. 

* subversion/libsvn_fs_fs/cached_data.c
  (block_read): Before checking the pack status, re-read it from disk.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1584374 13f79535-47bb-0310-9956-ffa450edef68