Version Control With Subversion Source

* en/book/ch06-server-configuration.xml
Remove markup from section <title>.

* en/book/ch04-branching-and-merging.xml
* en/book/ch09-reference.xml
Rename some busted section id's (s/branchemerge/branchmerge/).

Merge from ^/trunk/en r4392 (followup to r4387) and r4393 (new
'To Branch or Not to Branch?' section).

Finish isse 173 ("ch04: Consider noting when *not* to use branches").

* en/book/ch04-branching-and-merging.xml
(svn.branchmerge.when): New section cover the pros and cons of
branching.

* en/book/ch00-preface.xml
(svn.intro.righttool): Rework the "comparison" paragraph to point
out why the DVCS tools do well what they do well, and try to note
that centralized systems allow mixed-rev working copies without
using the terms "mixed-rev" or "working copies" (which aren't yet
introduced). Sheesh.

Suggested by: danielsh

Merge from ^/trunk/en r4390 (DAV proxy instructions disclaimer).

* en/book/ch06-server-configuration.xml
(svn.serverconfig.httpd.extra.writethruproxy): Note that this
section describes only one of several different possible proxy
configurations.

Suggested by: Dave Lawrence <dlawrence{__AT__}ad-holdings.co.uk>

Merge from ^/trunk/en r4386 (HTML stylesheet changes).

Merge from ^/trunk/en r4387 (moderize the '... Right Tool?' section).

Finish issue 65 ("Please talk about the cases where a true DVCS like
Git would be better than SVN") to the degree that I plan to do so.

* en/book/ch00-preface.xml
(svn.intro.righttool): Rework this section to make a distinction
between "choosing version control" and "choosing Subversion", and
then try to make an honest attempt to quickly explain how
Subversion differs from the DVCS tools and why one might (or might
not) choose it over such a tool.