Tweak the recording of merge tree conflicts to produce the right details when
a merge replaces a directory with a file, or the other way around.

* subversion/libsvn_client/merge.c
  (record_tree_conflict): Copy left_source from the old conflict if one exists.
  (merge_file_opened): Always replace the conflict from this function, for
    replacement conflicts, just like merge_dir_opened.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1443470 13f79535-47bb-0310-9956-ffa450edef68