Stop producing duplicate mergeinfo notifications when a merge adds a
subtree with explicit mergeinfo on it.

* subversion/libsvn_client/merge.c

  (record_mergeinfo_for_dir_merge): If we already recorded mergeinfo for a
   subtree remove that subtree from the hash we'll later pass to
   record_mergeinfo_for_added_subtrees(), since that function will
   effectively do the same thing, resulting in two mergeinfo notifications
   for the same path. 

* subversion/tests/cmdline/merge_tests.py

  (no_self_referential_filtering_on_added_path): Don't expect duplicate
   mergeinfo notifications.  Add a note referencing issue #4309 which is
   evident here.

  (merge_adds_subtree_with_mergeinfo): Don't expect duplicate
   mergeinfo notifications. 


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1440682 13f79535-47bb-0310-9956-ffa450edef68