This branch implements the 'invoke-diff-cmd' feature and is located at
https://svn.apache.org/repos/asf/subversion/branches/diff-cmd-feature/

This branch adds a new syntax facility to --diff-cmd and it's config
counterpart, whilst preserving the old behaviour.

The new syntax allows the use of 4 keywords: 

    %svn_new_label, %svn_old_label, %svn_old %svn_new 

At a minimum, %svn_new and %svn_old are required amongst any arbitrary
character sequence.

The code looks for the presence of %svn_old and if it does not detect
it, it translates the --diff-cmd input and --extensions if they have
been set to match the old --diff-cmds' translation before parsing the
result with svn_io__create_custom_diff_cmd().


Usage:
======

Examples:

svn diff --diff-cmd=diff %svn_new %svn_old
svn diff --diff-cmd=diff -L %svn_new_label  %svn_new -L "Old File" %svn_old


TODO:
=====

1) Finalise the current parsing capabilities
2) Write a fuzzing test for svn_io__create_custom_diff_cmd()
3) Rewrite the final svn_io__create_custom_diff_cmd() design
4) Solve the "diff\ script" problem
5) Solve the empty single back quotes problem
6) Discuss if adding this to the existing --diff-cmd (as per 2044)
   is useful and desired.


Problems:
=========

1) The "diff\ script" problem test problem:

   /subversion/tests/cmdline/diff_tests.py 52 

fails, because whilst the output is as expected, the shell fails to
find the "diff script".

2) The empty single back quotes problem:

Emtpy single quotes are sent to the shell duplicated (twice it seems).

The command: 

./subversion/svn/svn diff --diff-cmd="diff --changed-group-format='%<'\
 --unchanged-group-format='' %svn_new %svn_old"

Causes the external diff command to display 4 backticks in the
unchanged-group-format, and where there should be no line at all,
"''''" (4 backticks) are displayed.