git-svn: use an 'anonsvn' username and 'anonsvn' password

git-svn: use an 'anonsvn' username and 'anonsvn' password

For sites that require a password but support anonymous fetching
a username of 'anonsvn' and a password of 'anonsvn' is commonly used.

So instead of supplying nothing, use anonsvn and anonsvn.  More
sites will work and by explicitly always supplying a username no
user name from the system itself will ever be sent.