Checking if a proposed name is appropriate for a branch is strictly
a subset of checking if we want to allow creating or updating a
branch with such a name. The mysterious sounding 'attr_only'
parameter to validate_new_branchname() is used to switch the
function between these two roles.
Instead, split the function into two, and adjust the callers. A new
helper validate_branchname() only checks the name and reports if the
branch already exists.