On the fsfs-format7: introduce two rev / pack file utility functions

* subversion/libsvn_fs_fs/util.h
  (packed_base_rev,
   pack_size): declare new utilities
* subversion/libsvn_fs_fs/util.c
  (packed_base_rev,
   pack_size): implement them


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1476634 13f79535-47bb-0310-9956-ffa450edef68