Fix issue #4138 'replacement in merge source not notified correctly'.

* subversion/libsvn_client/merge.c

  (remove_source_gap): New.

  (notify_merge_begin,
   record_mergeinfo_for_dir_merge): Remove gap revisions from merge
   notifications.

* subversion/tests/cmdline/merge_tests.py

  (merge_source_with_replacement): Remove XFail decorator, adjust failure
   status comments, and expand test to cover a reverse merge.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1453635 13f79535-47bb-0310-9956-ffa450edef68