Version Control With Subversion Source

merging a branch back into the trunk. Fitz, cmpilato... any chance we
could upload a new version to the svnbook.red-bean.com? :-)

* ch04.xml: add a new subsection to the chapter on 'svn merge', and do
a tiny bit of reorg on other subsections. describe the common
use-case of merging an entire branch back into trunk.