Fix issue #4137 'redundant notifications in single editor drive merge'.

* subversion/libsvn_client/merge.c

  (find_nearest_ancestor_with_intersecting_ranges): New.

  (notify_merge_begin): Use find_nearest_ancestor_with_intersecting_ranges
   rather than the more naive find_nearest_ancestor.

* subversion/tests/cmdline/merge_tests.py

  (single_editor_drive_merge_notifications): New test for the issue.

  (test_list): Add single_editor_drive_merge_notifications.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1445029 13f79535-47bb-0310-9956-ffa450edef68