Version Control With Subversion Source

any of the 3 usage patterns' and issue #134 'ch09: merge
reference should mirror changes made to 'svn merge --help'
text in 1.7'.

* en/book/ch09-reference.xml

(svn.ref.svn.sw): Note --ignore-ancestry's rather important
effect on 'svn merge' beyond what it shares with 'svn diff'.

(svn.ref.svn.c.merge): Rework 'svn merge' reference to more
closely track the revised 'svn merge --help' text for 1.7.