On the 'move-tracking-2' branch: Add some assertions.
On the 'move-tracking-2' branch: Add some assertions.

* subversion/libsvn_delta/element.c
  (payload_invariants): New.
  (svn_element_payload_dup,
   svn_element_payload_equal): Assert validity as a precondition.
  (svn_element_payload_create_ref,
   svn_element_payload_create_dir,
   svn_element_payload_create_file,
   svn_element_payload_create_symlink): Assert validity as a postcondition.

git-svn-id: 486ec3464084f62cfe8608152673c827b947faa2@1682277 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion