fix(stack): preserve Change-Id when amending commits with -m flag

[jd]

When tools like Claude Code amend commits using git commit --amend -m "msg",
the Change-Id was being lost because:

1. The -m flag replaces the entire commit message
2. The commit-msg hook only preserves Change-Id if already present in the message

This adds a prepare-commit-msg hook that detects amend operations and preserves
the original Change-Id. The detection works by comparing GIT_AUTHOR_DATE (which
git sets to the original commit's author date during --amend) with HEAD's author
date. An additional check ensures the date is at least 2 seconds old to avoid
false positives when commits happen in quick succession.

Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com

[mergify]

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[mergify]

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[Copilot]

Pull request overview

This PR fixes an issue where Change-IDs were being lost when commits are amended using git commit --amend -m "message", a common pattern used by tools like Claude Code. The solution adds a new prepare-commit-msg git hook that detects amend operations and preserves the original Change-ID.

Changes:

• Refactored hook installation logic in setup.py to support multiple hooks
• Added new prepare-commit-msg hook to preserve Change-IDs during amends with -m flag
• Added comprehensive integration tests covering various commit and amend scenarios

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
mergify_cli/tests/stack/test_setup.py	Added integration tests for hook behavior with real git repository, including tests for new commits, amends with/without -m flag, and Change-ID preservation
mergify_cli/stack/setup.py	Refactored hook installation into reusable _install_hook() function to support installing both commit-msg and prepare-commit-msg hooks
mergify_cli/stack/hooks/prepare-commit-msg	New shell script hook that detects amend operations by comparing author dates and preserves Change-ID from HEAD when amending with -m/-F flags

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Merge Queue Status

✅ The pull request has been merged at 00548c0

This pull request spent 2 minutes 21 seconds in the queue, including 1 minute 47 seconds running CI.
The checks were run on draft #910.

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• all of [🛡 Merge Protections rule Enforce conventional commit]:
  • title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?:
    • fix(stack): preserve Change-Id when amending commits with -m flag #903
• all of [🛡 Merge Protections rule 👀 Review Requirements]:
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      • fix(stack): preserve Change-Id when amending commits with -m flag #903
    • author = dependabot[bot]
      • fix(stack): preserve Change-Id when amending commits with -m flag #903
    • author = mergify-ci-bot
      • fix(stack): preserve Change-Id when amending commits with -m flag #903
    • author = renovate[bot]
      • fix(stack): preserve Change-Id when amending commits with -m flag #903
• all of [🛡 Merge Protections rule 📕 PR description]:
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    • fix(stack): preserve Change-Id when amending commits with -m flag #903
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