On the invoke-diff-cmd branch: rewrite __create_custom_diff_cmd and
it's python test to preserve strings inside the command string that is
expanded.  

* subversion/libsvn_subr/io.c

  (__create_custom_diff_cmd): Preserve internal strings that have
    spaces as one arg for result[].

  (svn_io_run_diff2): Add advice where to find the test for this
    function.

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


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1531371 13f79535-47bb-0310-9956-ffa450edef68