On the fsfs-format7 branch:  fix a number of block read inefficiencies.
When reading data,  make sure we read everything from that block (fix
for root nodes and block read).  Keep blocks properly aligned such that
they remain aligned to the underlying device's block structure.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__rev_get_root): trigger block read just as a normal noderev
   read access would
  (block_read_rep_header): skip the parser if the data is already in cache
  (block_read_windows): ditto; also ensure proper read block alignment
  (block_read): when reading across a block boundary, read everything
   from the last block as well but don't repeat this process

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1446121 13f79535-47bb-0310-9956-ffa450edef68