branch: split validate_new_branchname() into two

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.

This loses one NEEDSWORK from the branch API.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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