Fix the bug introduced in r27494, the bug is 'multiple begin merge notifications i.e for every operative merge on any node results in  a merge-beging notification.'
Fix the bug introduced in r27494, the bug is 'multiple begin merge
notifications i.e for every operative merge on any node results in 
a merge-beging notification.'

* subversion/libsvn_client/merge.c
  (notification_receiver): Notify 'merge-begin' notification only once.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion