On the fsfs-format7 branch:  "flatten" representation_t by replacing
the svn_checksum_t* sub-structures with plain digests.  That makes
representations and noderevs much easier to cache and copy.

Only in a few places do we have to construct proper svn_checksum_t
objects, most changes are simple adaptions and some are simplifications.
The code would be even nicer if we had access to lib_subr's private
MD5 and SHA1 interfaces.

* subversion/libsvn_fs_fs/fs.h
  (representation_t): replace checksum objects with digests

* subversion/libsvn_fs_fs/cached_data.c
  (rep_read_baton,
   delta_read_baton): replace md5 checksum objects with digests
  (rep_read_get_baton,
   svn_fs_fs__get_file_delta_stream): update
  (delta_read_md5_digest): simplify
  (rep_read_contents): construct local checksum object from baton

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__file_checksum): update
  (svn_fs_fs__rep_copy): simplify

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__parse_representation): update
  (format_digest): replaces DISPLAY_MAYBE_NULL_CHECKSUM macro
  (svn_fs_fs__unparse_representation): use the new function

* subversion/libsvn_fs_fs/rep-cache.c
  (svn_fs_fs__walk_rep_reference,
   svn_fs_fs__get_rep_reference): update 
  (svn_fs_fs__set_rep_reference): construct local checksum object

* subversion/libsvn_fs_fs/temp_serializer.c
  (serialize_checksum,
   deserialize_checksum): drop obsolete functions
  (serialize_representation,
   svn_fs_fs__noderev_deserialize): simplify

* subversion/libsvn_fs_fs/transaction.c
  (store_sha1_rep_mapping,
   digests_final,
   write_final_rev): update
  (get_shared_rep): update; construct temporary checksum object

* subversion/libsvn_fs_fs/util.h
  (path_txn_sha1): take a digest instead of a checksum

* subversion/libsvn_fs_fs/util.c
  (path_txn_sha1): construct temp. checksum object

* tools/server-side/svn-rep-sharing-stats.c
  (value_t): make SHA1 checksum an embedded member
  (record,
   pretty_print): update


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1462727 13f79535-47bb-0310-9956-ffa450edef68