Merge branch 'jc/branch-name-sanity'

"git branch" and "git checkout -b" are now forbidden from creating a branch whose name is "HEAD". * jc/branch-name-sanity: builtin/branch: remove redundant check for HEAD branch: correctly reject refs/heads/{-dash,HEAD} branch: split validate_new_branchname() into two branch: streamline "attr_only" handling in validate_new_branchname()