On the invoke-diff-cmd branch: Refactor __create_custom_diff_cmd() to
use the new delimiter schema.  Update svn_io_run_diff2(), add a
scratch_pool to svn_io_run_external_diff(), update help into in
svn_cl__options[].

* BRANCH-README:
  
  (What --invoke-diff-cmd and --invoke-diff3-cmd provide): Update svn
    help diff info.

* subversion/libsvn_subr/io.c

  (__create_custom_diff_cmd): Remove shell escape facility.  Change
    delimiters.

  (svn_io_run_diff2): Change call to __create_custom_diff_cmd to use
    the new delimiter schema.

  (svn_io_run_external_diff): Add scratch_pool and ensure destruction
    thereof.

* subversion/svn/svn.c

  (svn_cl__options[]): Update help info to reflect the new delimiter
    schema.

* subversion/tests/cmdline/diff_tests.py:
  
  (diff_invoke_external_diffcmd): Update test to use the new delimiter
    schema.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/invoke-diff-cmd-feature@1533721 13f79535-47bb-0310-9956-ffa450edef68