On the fsfs-format7 branch:  Prevent the packed number stream from
prefetching across block boundaries.

* subversion/libsvn_fs_fs/index.c
  (stream_error_create): new error construction utility
  (packed_stream_read): limit the prefetch close to block boundaries

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1446113 13f79535-47bb-0310-9956-ffa450edef68