Fix issue #34 - reporting protected refs with slashes in the name

Eg this crashed:

$ java -jar bfg-1.11.1.jar --strip-blobs-bigger-than 100K -p 'feature/worker-prototype' my-repo.git

The code wasn't expecting branch names with a slash (because, like,
obviously those are ug-lee) - and it's not possible to create filenames
that contain slashes:

https://github.com/rtyley/bfg-repo-cleaner/issues/34