Version Control With Subversion Source

* src/en/book/ch-branching-and-merging.xml:

- a bunch of misc cleanups/rewordings
- don't mention CVS attic junk
- don't mention the specific command 'svn obliterate', keep it vague
- mention pegrev syntax where we can
- don't talk about svn 1.0, that's so 2004.
- add new section warning about problems with merging moves/copies