feat(stack): add revision history comments for force-pushed PRs

[jd]

When stack push force-pushes a PR branch, reviewers have no way to see
what changed between the previous and new commits. This adds a "Revision
History" comment on each updated PR showing all push iterations with:

• Compare links (github.com/owner/repo/compare/old...new) so reviewers
  can see exactly what changed
• Change type labels (content vs rebase) using git patch-id to detect
  whether the force-push changed actual code or just rebased
• Timestamps for each push iteration

The feature is enabled by default. Disable per-push with
--no-revision-history or globally with:
git config --global mergify-cli.stack-revision-history false

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

[mergify]

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[Copilot]

Pull request overview

Adds an opt-out “Revision history” PR comment feature to stack push so reviewers can see what changed across force-push iterations (including compare links, change-type detection via patch-id, and timestamps).

Changes:

• Add git-config-backed default for revision history behavior (mergify-cli.stack-revision-history).
• Add revision-history detection + comment create/update flow to stack push.
• Add/adjust tests to cover revision history defaults, change-type detection, and comment creation/update behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
mergify_cli/utils.py	Adds get_default_revision_history() to read the new git config default.
mergify_cli/stack/push.py	Implements patch-id based change-type detection, fetching old PR heads, and posting/updating “Revision history” comments.
mergify_cli/stack/cli.py	Adds --no-revision-history option wired to the config-driven default.
mergify_cli/tests/conftest.py	Updates shared git mock to include the new config key.
mergify_cli/tests/stack/test_push.py	Adds extensive unit/integration-style coverage for the new revision history feature paths.

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