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merge: respect parents order when using `graft` on a merge

Authored by marmoute.

Description

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.