merge: respect parents order when using `graft` on a merge The previous code did not record the index of the replaced parent. It was always using the "graft" destination as `p1`. This could switch parents order in some situation (eg: some of the evolve evolving merge case). Recording and using the information fixes the issue in evolve. We are not aware of core commands calling graft in that fashion, so we could not build a simple test case for it using core commands.