Fix issue #4348, "missing directory replace on update 1.8 svnserve 1.7 client".

* subversion/svnserve/serve.c
  (update): Correct the backward-compatibility code for the
    'ignore_ancestry' parameter introduced in r1465292.

Found by: philip

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1466659 13f79535-47bb-0310-9956-ffa450edef68