Tweak regression test added in r1511273 in order to clarify the observed
problem, after some experimentation.

* subversion/tests/cmdline/diff_tests.py
  (diff_prev_copy_in_mixed_rev_parent): Rename to ...
  (diff_prev_of_copy): .. this. Show that the observed problem is triggered
   by copying across an unrelated revision, and is not rooted in mixed-rev
   working copy handling as previously suspected.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1511278 13f79535-47bb-0310-9956-ffa450edef68