Issue 59 in svnbook: 'svnsync init' supports a --allow-non-empty flag in Subversion 1.7

[svnbook at googlecode.com]

Comment #2 on issue 59 by cmpil... at gmail.com: 'svnsync init' supports a  
--allow-non-empty flag in Subversion 1.7
http://code.google.com/p/svnbook/issues/detail?id=59

Added the Reference bits for this in r4004.