Fix Issue #4408: diff --summarize includes duplicate path elements.

The path provided to the summarize_function needs to be relative to the target.
Most of the time the target is "" but if a path doesn't exist on one side
of the diff (add or delete) then the root of the editor drive is moved outside
the target and the target gets duplicated in the output.

This issue is fixed on 1.8.x and newer as diff --summarize has a new and
different implementation.  That implementation is not appropriate to backport
as it adds new features.  Also note that 1.8.x+ already has tests for these
cases.

* subversion/libsvn_client/repos_diff_summarize.c
  (create_item_baton): Use svn_relpath_skip_ancestor instead of the hack added
    to fix Issue #2765.
  (diff_deleted_dir, delete_entry): Skip the target.

* subversion/tests/cmdline/diff_tests.py
  (basic_diff_summarize): Add tests for a newly added file and dir.
  


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.7.x-issue4408@1512154 13f79535-47bb-0310-9956-ffa450edef68