Notify merge begin notification only if we have an operative merge.
Notify merge begin notification only if we have an operative merge.

* subversion/libsvn_client/merge.c
  (notification_receiver): Notify merge begin notification only if we have 
   an operative merge for three-way directory merge case.(Follow-up to r27191).
  (do_merge): Remove notification code for three-way merge.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@867568 13f79535-47bb-0310-9956-ffa450edef68

wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion