Restore the 1.8 behavior of svn_fs_contents_changed() and _props_changed(),
and switch all calling sites of the new API, svn_fs_contents_different() and
_props_different() back to using the old functions.

There were no user-visible problems associated with the old code.  The new
API doesn't improve any real use cases in the current code, but is causing
problems:

- We had a problem with misbehaving svn blame -g
  (http://svn.haxx.se/dev/archive-2015-06/0069.shtml, "Blame behaviour
   change in 1.9").

- We have an issue with repositories behaving differently in client-side
  operations like 'svn log' after dump/load
  (http://svn.haxx.se/dev/archive-2015-09/0269.shtml, "No-op changes no
   longer dumped by 'svnadmin dump' in 1.9"; also see issue #4598 in
   https://issues.apache.org/jira/browse/SVN-4598).

- We could experience same problems originating from other callers of the
  new API, because the low level behavior change associated with switching
  to it propagates up to higher levels like svn_repos or svn_ra and alters
  the behavior of many different callers like svn_ra_get_file_revs2() or
  the update reporter.  Third-party API callers could not be ready for it
  as well, because public API functions like svn_ra_get_file_revs2() didn't
  receive the erratum or a bump.

See the discussion in http://svn.haxx.se/dev/archive-2015-10/0022.shtml
("Re: No-op changes no longer dumped by 'svnadmin dump' in 1.9").

* subversion/libsvn_fs_base/dag.c
  (svn_fs_base__things_different): Compare the uniquifiers, as we did in 1.8.

* subversion/libsvn_fs_base/fs.h
  (node_revision_t.data_key_uniquifier): Remove the comment about not using
   this field.

* subversion/libsvn_fs_fs/fs_fs.c
  (svn_fs_fs__noderev_same_rep_key): Reintroduce this helper function.
  (svn_fs_fs__file_text_rep_equal, svn_fs_fs__prop_rep_equal): Always
   assume the strict mode in these helpers.

* subversion/libsvn_fs_fs/fs_fs.h
  (svn_fs_fs__noderev_same_rep_key): Declare the reintroduced helper.
  (svn_fs_fs__file_text_rep_equal, svn_fs_fs__prop_rep_equal): Update the
   docstrings for these helper functions.

* subversion/libsvn_fs_fs/dag.c
  (svn_fs_fs__dag_things_different): Preserve the current comparison behavior
   in strict mode.  Restore the 1.8 way of comparing the representation keys
   in non-strict mode.

* subversion/libsvn_fs_fs/tree.c
  (merge): Restore the 1.8 way of comparing property lists.

* subversion/libsvn_fs_fs/fs.h
  (representation_t.uniquifier): Remove the comment about not using this
   field.

* subversion/libsvn_repos/delta.c
  (delta_proplists): Switch back to using svn_fs_props_changed().
  (svn_repos__compare_files): Restore this function to its 1.8 state.
  (delta_files): Restore the 1.8 way of comparing files.

* subversion/libsvn_repos/dump.c
  (dump_node): Switch back to using svn_fs_contents_changed() and
   svn_fs_props_changed().

* subversion/libsvn_repos/reporter.c
  (delta_proplists): Switch back to using svn_fs_props_changed().

* subversion/libsvn_repos/rev_hunt.c
  (send_path_revision): Switch back to using svn_fs_contents_changed().
   Remove the no longer necessary hack for svn blame -g with older clients.

* subversion/include/svn_ra.h
  (svn_ra_get_file_revs2): Remove the 1.9-specific behavior note.

* subversion/include/svn_repos.h
  (svn_repos_get_file_revs2): Remove the 1.9-specific behavior note.

* subversion/tests/cmdline/svnadmin_tests.py
  (dump_no_op_change): No longer fails with fsfs and bdb.