Tell people who pass mixed target types to 'svn diff' synopsis 1 that they
are probably looking for one of the '--old X --new Y' invocations.

Suggested by: Alexey Neyman

* subversion/svn/diff-cmd.c
  (svn_cl__diff): Wrap the error from svn_cl__assert_homogeneous_target_type()
   with a somewhat verbose but informative message.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1442645 13f79535-47bb-0310-9956-ffa450edef68