tree: Use normalized node IDs in fields

[alex-pardes]

Description

When composing changesets, ModularChangeFamily was using change2's node IDs as canonical, while using change1's node IDs in the composed fields. This PR changes the implementation to use change1's node IDs as canonical, so the composed field changesets will contain the canonical IDs.

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[github-actions]

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Base commit: unresolved
Head commit: 64958f4173858d80eef9c1651bf27ceaf9f0c972

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