Follow-up to r1604933.

* subversion/libsvn_fs_fs/fs_fs.c
  (read_global_config): Make block-read actually opt-in, i.e. default
                        to "off" if nobody specified the option.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1606656 13f79535-47bb-0310-9956-ffa450edef68