When opening a repository diff report the root directory as opened to
the diff tree processor.

This fixes a case where the tree-tee processor would segfault, and a case where
the merge code didn't detect a tree conflict in the right place.

* subversion/libsvn_client/repos_diff.c
  (open_root): Also report opening the root; not just closing.

* subversion/tests/cmdline/merge_tests.py
  (merge_target_selection): Expect tree conflict on the invalid target instead
    of below the target. This finally makes the file and directory case
    behave in the same way.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1440379 13f79535-47bb-0310-9956-ffa450edef68