I wrote a tool a while ago (4 years? my, time flies) that does exactly what's requested there (bisects across merge commits): https://github.com/ayust/misc/blob

I wrote a tool a while ago (4 years? my, time flies) that does exactly what's requested there (bisects across merge commits): https://github.com/ayust/misc/blob/master/bisect-merges https://github.com/ayust/misc/blob/master/bisect-merges