Make our authz_read callback handle paths in uncommitted txns.

* mod_dav_svn/update.c (authz_read): if a txn_root is passed in,
  authorize the "copied_from" rev/path, not the literal path in the txn.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/partial-authz-dev@847745 13f79535-47bb-0310-9956-ffa450edef68