Version Control With Subversion Source

* src/en/book/ch04.xml:

- don't use HEAD in merge command examples, it's not really a
good practice to encourage.

* src/en/book/ch06.xml:

- mention pre-commit hooks as an option for controlling per-dir
write access with svnserve.

- Merge the too-short "external path-based authz" section into
this paragraph.

- (Servers and Permissions: A Word of Warning): remove
assumption that all repositories are BDB-based.

* src/en/TODO: remove some TODOs.
also, move a TODO from me to fitz, since it's chapter 3.