Prefer complete merged changes when backporting squash-merged PRs

[svick]

Fixes #17205.

The backport workflow currently replays every original PR commit using git am. Intermediate commits can conflict with the target branch even when the final merged change applies cleanly.

This change:

• Compares a one-parent merged commit's diff with the complete PR diff using git patch-id --verbatim.
• Cherry-picks the merged commit when it is verified to represent the complete change.
• Retains git am for multi-commit rebases, two-parent merge commits, unverifiable cases, and workflows specifying additional git am switches.
• Fetches refs/pull/<number>/head before git am, providing objects required for three-way application of patches from forks.
• Falls back safely if inspecting or cherry-picking the merged commit fails.

The production workflow script was exercised in dry mode against squash, merge-commit, rebase, missing-object, custom-switch, and conflict scenarios. Here, "dry mode" means the actual inline script ran against temporary local Git repositories with mocked GitHub APIs and intercepted pushes.

Note

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Pull request overview

This PR updates the reusable backport workflow to avoid replaying intermediate PR commits (via git am) when the final merged PR change can be safely applied as a single commit, reducing unnecessary conflicts—especially for squash-merged PRs and PRs from forks.

Changes:

• Downloads both the full mailbox patch series and the complete PR diff, then compares patch-ids to verify whether the merged commit represents the full PR change.
• Preferentially cherry-picks the merged commit when verification succeeds, otherwise falls back to git am.
• Fetches refs/pull/<number>/head before git am to improve three-way patch application when PR objects aren’t present locally.

Comments suppressed due to low confidence (1)

.github/workflows/backport-base.yml:304

• merged_diff_file is not guaranteed to exist (e.g., no merge_commit_sha, or merge commit with >2 parents, or verification skipped). git reset HEAD -- ... ${merged_diff_file} will fail if the file is missing, which can break the conflict-resolution flow after a failed git am. Make the reset conditional on the file existing.

              await exec.exec(`git reset HEAD -- ${patch_file} ${diff_file} ${merged_diff_file}`);

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