Issue "New Branch" shouldn't include full title of confidential issue

Right now the branch title will be possibly-sensitive-issue-title-123 where 123 is the issue ID.

For confidential issues, it should be a simple issue-123.

Of course the commits themselves could still give the vulnerability away, but chances are relatively small people will read every commit and branch, and commits can be carefully named to be technically accurate but not give away that it fixes a vulnerability

cc @zj