[PATCH]: Fix up diff against schedule deleted output

[Daniel Berlin <db...@dberlin.org>]

The attached fixes up the schedule deleted diff output so that it
matches what people expect (IE an empty diff with the correct header).

I've also added a regression test to verify the output.

[[[
   Fix diff of add against schedule delete output so that it displays an
   empty diff with the correct header.  Add regression test to verify
   output.

   * subversion/libsvn_wc/diff.c
     (delete_entry): Push use of empty file down into code farther
     so that we can still display the correct header, and mark
     the diff as done.

   * subversion/tests/clients/cmdline/diff_tests.py
     (diff_schedule_deleted): New regression test.
]]]