On the invoke-diff-cmd branch: Refactor __create_custom_diff_cmd() and
it's test.  Remove svn_io_create_custom_diff_cmd().

* subversion/include/svn_io.h
  (svn_io_create_custom_diff_cmd): Remove.  
    
* subversion/libsvn_subr/io.c
  (__create_custom_diff_cmd): Refactor.
  (svn_io_create_custom_diff_cmd): Remove.

* subversion/tests/cmdline/diff_tests.py
  (diff_invoke_external_diffcmd): Adjust tests for the above behavior.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1531530 13f79535-47bb-0310-9956-ffa450edef68