Version Control With Subversion Source

and subtree mergeinfo.

* en/book/ch04-branching-and-merging.xml
(svn.branchemerge.basicmerging.stayinsync.subtree): New sidebar
covering the basics of subtree merges and subtree mergeinfo.

(svn.branchmerge.basicmerging.mergeinfo.inheritance): Add an id
attribute to the sidebar on mergeinfo inheritance, so we can
reference it from the new sidebar above.