Fix issue #3247 'merge doesn't honour specified SOURCE'.

* subversion/svn/svn.c
  (svn_cl__cmd_table): Accurately describe the current 'svn merge' behavior
   for implied merge targets.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1458931 13f79535-47bb-0310-9956-ffa450edef68