Replace weird code that uses magic numbers and produces warnings
in maintainer mode with equally weird code that does not use magic
numbers and avoids the warnings.

* subversion/libsvn_delta/svndiff.c
  (SVNDIFF_V0, SVNDIFF_V1): New; svndiff header represenations.
  (SVNDIFF_HEADER_SIZE): New.
  (write_handler): Use the above new stuff instead of magic
   numbers and string literals.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619075 13f79535-47bb-0310-9956-ffa450edef68