Version Control With Subversion Source

* en/book/ch03-advanced-topics.xml,
* en/book/ch04-branching-and-merging.xml,
* en/book/ch05-repository-admin.xml,
* en/book/ch06-server-configuration.xml
Try another approach to fixing links to Reference sections, adding
the text 'in <xref linkend="svn.ref"/>' after each such reference in
the book that points to an ambiguously named piece of the Reference.