(Basically) eliminate the remaining CPU processing overhead of FSFS
format 7 over format 6 in non-block-read mode.

We simply don't prefetch index data in non-block-read mode anymore.
Since we will only ever read index data when we had to open the file
anyway to read some of its content, reading non-cached index parts
is relatively cheap (seek and read operations only).

* subversion/libsvn_fs_fs/index.c
  (l2p_index_lookup,
   p2l_index_lookup): Make index content prefetch contingent upon
                      block-read being enabled.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606662 13f79535-47bb-0310-9956-ffa450edef68