Is saying "bastagh" instead of "Astaghfirullah" (I ask Allah for forgiveness) and then denying it by saying "bistu" considered a joke, and does it amount to disbelief?

It is not permissible to joke with religious words. One must show reverence towards them, and repentance for doing so is obligatory. What was said could be a mockery of the word "bastakh" and not of the legal act of seeking forgiveness (Istighfar). Even with this possibility, it does not constitute apostasy (kufr). We advise you to occupy yourself with what concerns you and to divert your mind from the whispers of declaring others disbelievers.