Reset before merge with commits

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%D0%91%D0%B5%D0%B7-%D0%B8%D0%BC%D0%B5%D0%BD%D0%B8-1
%D0%91%D0%B5%D0%B7-%D0%B8%D0%BC%D0%B5%D0%BD%D0%B8-1935×848 24.1 KB

How can I completely and irrevocably delete everything before the merge? To remain two disjoint branches