Improve fix for Issue #4400.

* subversion/libsvn_ra_serf/commit.c
  (checkout_dir): Use svn_relpath_skip_ancestor()
    Add an SVN_ERR_ASSERT on parent->working_url.

Suggested by: danielsh
(the SVN_ERROR_ASSERT)


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1512195 13f79535-47bb-0310-9956-ffa450edef68