Fix Issue #4400: commits fail with copy and deep delete under copy.

* subversion/libsvn_ra_serf/commit.c
  (checkout_dir): Generate the working_url when a parent is added by combining
    the add parent's working_url and the unique (vs add parent) portion of the
    checkout directory's relpath.

* subversion/tests/cmdline/commit_tests.py
  (commit_cp_with_deep_delete): Remove the XFail decorator from the test which
    will pass now.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507589 13f79535-47bb-0310-9956-ffa450edef68