If you pass the --prefix=svn/ flag to the git svn clone command, then all of the Subversion branches would be named like remotes/svn/branchname. If this is acceptable to you, it fixes the “refname is ambiguous” warning. It also gives you a nice way of referring to the remote svn branches, as in for instance if you want to create a local tracking branch it would be something like:
$ git checkout -b branchname svn/branchname
The local branch then has the same name as the remote svn branch, and no ambiguous refname problem.