How do I tell git-svn about a remote branch created after I fetched the repo?

You can manually add the remote branch,

git config --add svn-remote.newbranch.url https://svn/path_to_newbranch/
git config --add svn-remote.newbranch.fetch :refs/remotes/newbranch
git svn fetch newbranch [-r<rev>]
git checkout -b local-newbranch -t newbranch
git svn rebase newbranch