Keep '--- Merging *' and '--- Recording mergeinfo *' notifications
consistent when a file merge is broken into multiple editor drives.

* subversion/libsvn_client/merge.c

  (do_file_merge): Handle the case where what is actually merged is a
   subset of the requested merge source.

* subversion/tests/cmdline/merge_tests.py

  (conflicted_split_merge_with_resolve): Remove workaround introduced in
   r1459012.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1459895 13f79535-47bb-0310-9956-ffa450edef68