The problem with that scenario is that an extension to Art 50 requires the agreement of the other 27 EU members, and they've already said that they will only agree to that if the extension is for a) UK Parliament to sign off on the current WA or b) a change in the UK Gov negotiating position (removing or altering the UK-defined red lines enough to mean that a significantly different WA can be agreed. They may agree in the case of a referendum on the final deal, but if a General Election were to be called, there's absolutely no guarantee that the EU27 will agree to an extension. A revocation of Art 50 requires only the UK Government.