git-commit-tree: ignore duplicate parents

But warn about them.  If somebody really ends up later wanting to
explicitly add a note that something has the same parent twice (who
knows, there are strange people around), we can add a flag to say that
it's expected and ok.

This was brought on by a commit in the kernel tree, where a repeated
merge caused a duplicate parent.

Parent duplicates aren't "wrong" per se, they're just in practice not
something you are ever interested in.

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