On the fsfs-format7 branch:  Add a function to determine the on-disk size
as well as the size of the reconstructed fulltext of a given representation
(without having a suitable representation_t).

* subversion/libsvn_fs_fs/temp_serializer.h
  (svn_fs_fs__txdelta_cached_window_t): add start offset as well so that
                                        we can determine the on-disk size

* subversion/libsvn_fs_fs/cached_data.h
  (svn_fs_fs__get_representation_length): declare new private API

* subversion/libsvn_fs_fs/cached_data.c
  (window_sizes_t): new aux type
  (get_cached_window_sizes_func,
   get_cached_window_sizes): new functions to determine window sizes for
                             cached txdelta windows
  (set_cached_window,
   read_delta_window): cache window start offset as well
  (init_rep_state,
   cache_windows): factored out from block_read_windows
  (block_read_rep_header): rename to ...
  (read_rep_header): ... this one
  (svn_fs_fs__get_representation_length): implement
  (block_read_windows): update
  (block_read_contents): update caller

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1490560 13f79535-47bb-0310-9956-ffa450edef68