test: pin hybrid chaining when the hardcoded parent is not the head

[jd]

A hybrid whose down_revision points at a revision that is no longer
the head is spliced in behind it, re-parenting the revisions in between.
That keeps the chain linear and single-headed, which is the property
this library exists to provide, but nothing pinned it -- so a future
change could turn it into a fork without a test noticing.

Two cases, both intended:

• the parent was already stale when the file was written (an authoring
  error: the rule is to hardcode only onto the current head)
• the parent was the head when the file was written, and another branch
  merged first (prevented upstream by the merge queue, which tests the
  branch against the rebased base)

The second case is driven through real branches and real merge commits
rather than a hand-written git order, so it also covers
_get_git_commit_order producing the order that
test_dynamic_inserted_before_hybrid_no_multiple_heads assumes.

Both walk the reconstructed chain, so they fail loudly on a fork rather
than only on a changed dict.

Refs #47

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com

Claude-Session-Id: 934c0f4e-8a8c-425a-826e-ed4ae67d6d2d

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[Copilot]

Pull request overview

Adds regression coverage to ensure hybrid migrations whose hardcoded down_revision is not the current head are still spliced into the chain in a way that preserves a single, linear head, and to pin the git-add ordering expectations that existing hybrid/dynamic tests rely on.

Changes:

• Add a test that exercises a hybrid hardcoding a stale parent and verifies chain re-parenting keeps a single head (no fork).
• Add an integration-style test using real branches/merge commits to verify _get_git_commit_order produces the ordering assumed by the existing “dynamic inserted before hybrid” scenario.
• Validate chain linearity by walking the reconstructed full chain and failing loudly on forks/cycles.

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Merge Queue Status

• ✅ Entered queue — 2026-08-13 15:13 UTC · Rule: default · triggered by merge protections
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• ✅ Merged — 2026-08-13 15:13 UTC · at a7111f0b1dd769ac23595c521947a8399964c120 · squash

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