Before this commit, `clone` gave different results depending on whether the original object
had an attached singleton class or not.
Consider the following setup:
```
class Foo; end
Foo.singleton_class.define_method(:foo) {}
obj = Foo.new
obj.singleton_class if $call_singleton
clone = obj.clone
```
When `$call_singleton = false`, neither `obj.singleton_class.singleton_class` nor
`clone.singleton_class.singleton_class` own any methods.
However, when `$call_singleton = true`, `clone.singleton_class.singleton_class` would own a copy of
`foo` from `Foo.singleton_class`, even though `obj.singleton_class.singleton_class` does not.
The latter case is unexpected and results in a visibly different clone, depending on if the original object
had an attached class or not.
Co-authored-by: Ufuk Kayserilioglu <ufuk.kayserilioglu@shopify.com>
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