Speed up the metadata check for log. addressed FSFS repos.

* subversion/libsvn_fs_fs/cached_data.c
  (svn_fs_fs__check_rep): Reuse the rev / pack file through the same
                          hinting mechanism as in older formats.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1640062 13f79535-47bb-0310-9956-ffa450edef68