Fix passing nil to protected_tag?

Previously this method would directly receive the output of tag_name().
This method could either return a String or nil. In the previous setup
this would somehow magically work but because Rugged::TagCollection#[]
doesn't accept nil values it started to fail.

To work around this the elsif in change_access_check() assigns the
result of tag_name() to a local and then _only_ calls protected_tag?()
if the tag name is not nil. The extra parenthesis are put in place to
ensure that things are parsed correctly, without these the code would be
parsed as follows:

    elsif tag_ref = (tag_name(ref) && protected_tag(tag_ref))

During runtime this would basically resolve to:

    elsif tag_ref = (tag_name(ref) && protected_tag(nil))

This is because when you refer to the variable you're assigning _in_ the
assignment Ruby returns nil instead of raising an error.