Add some initial code to support creating git-like binary file diffs when
running svn diff --git.

Currently the output appears to be not 100% identical to that of git, but I'm
not sure if that is caused by different compression settings or by a bug in
the new code.

* subversion/include/svn_diff.h
  (svn_diff_output_binary): New function.

* subversion/libsvn_client/diff.c
  (diff_content_changed): Produce git binary diffs instead of an error when
    requested.

* subversion/libsvn_diff/binary_diff.c
  New file.
  (create_compressed,
   write_literal,
   svn_diff_output_binary): New functions.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1599552 13f79535-47bb-0310-9956-ffa450edef68
* subversion/libsvn_diff/binary_diff.c
  (write_literal): Perform allocation after variable definitions to avoid C89
    errors. Fix small indentation problem.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1599572 13f79535-47bb-0310-9956-ffa450edef68