On the invoke-diff-cmd branch: Add refactored version
__create_custom_diff_cmd as a handrolled O(n) alternative offering to
(old and suspect) svn_io_create_custom_diff_cmd. Update diff_tests.py
Nr. 49.

* subversion/include/svn_io.h

  (__create_custom_diff_cmd): New internal function.(temporary
  installation for comparison)

* subversion/libsvn_subr/io.c

  (__create_custom_diff_cmd): New temporary internal function,
    refactored copy of svn_io_create_custom_diff_cmd().

  (svn_io_run_external_diff): Change call temporarily from
    svn_io_create_custom_diff_cmd() to __create_custom_diff_cmd().
    Adjust code to conform to 80 char limit.  

 * subversion/tests/cmdline/diff_tests.py

  (diff_invoke_external_diffcmd): Fix typo in description, improve
    tests. 




git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1526539 13f79535-47bb-0310-9956-ffa450edef68