merge: respect parents order when using `graft` on a merge, this time for real See a4ca0610c754. potherp1 is a boolean variable that means "pother is ctx.p1", and parents is naturally [ctx.p1, ctx.p2]. pctx is always removed from parents, so if pctx is parents[0], then we end up using parents[1] as pother. To be true to its name, potherp1 should then be True only when pctx is at parents[1].