git-svn: document --username
git-svn: document --username

Also, it turns out that SVN::Ra doesn't attempt to deal with
authentication or pass the username to ssh when doing svn+ssh://
URLs

Signed-off-by: Eric Wong <8c2951295f8292b0c03727bc4652981172298eae@yhbt.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git