Stop 'svn diff' from printing the same error message twice when mixed
targets are supplied to 'svn diff' synopsis 1.

Instead of...
   svn: E205000: Cannot mix repository and working copy targets
   svn: E200009: Cannot mix repository and working copy targets
... the error message now says:
   svn: E205000: Error parsing arguments
   svn: E200009: Cannot mix repository and working copy targets

* subversion/svn/util.c
  (svn_cl__assert_homogeneous_target_type): Don't repeat the same error
   message as already returned by svn_client__assert_homogeneous_target_type().
   Use the default text for SVN_ERR_CL_ARG_PARSING_ERROR instead.

* subversion/tests/cmdline/input_validation_tests.py
  (invalid_delete_targets, invalid_diff_targets, invalid_lock_targets,
   invalid_unlock_targets, invalid_relocate_targets): Adjust test expectations.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442662 13f79535-47bb-0310-9956-ffa450edef68