Add a test for Issue #4421 to trunk: deep adds of diff --summarize target.

Trunk hasn't had this problem for a while, but add the test for it so if
we ever regress here we'll know.

* subversion/tests/cmdline/diff_tests.py
  (basic_diff_summarize): Test doing diff --summarize on a target that even the
    parent doesn't exist in the base revision.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1535591 13f79535-47bb-0310-9956-ffa450edef68