D6151: branchmap: remove the dict interface from the branchcache class (API)

[Yuya Nishihara]

>      def copy(self):
>          """return an deep copy of the branchcache object"""
> -        return type(self)(
> -            self, self.tipnode, self.tiprev, self.filteredhash,
> +        return branchcache(
> +            self.entries, self.tipnode, self.tiprev, self.filteredhash,
>              self._closednodes)

In order to support subclassing, a `branchcache` type has to be resolved
dynamically. IIRC, that's the point of `type(self)` use here.