feat(stack): port replay git helpers (merge-tree + diff-tree) to Rust

[jd]

When stack push force-pushes an amended PR head, the revision-
history compare URL …/compare/<old>...<new> is useless when the
rebase moved the base — reviewers see the whole PR diff instead
of just the edit. Python fixes this by replaying the user's
amendment onto parent(old_sha) and uploading a synthetic
commit, so compare anchors on old_sha and renders only the
edit.

Port the git-only half of mergify_cli/stack/replay.py to
mergify-stack::replay:

• compute_merged_tree — git merge-tree --write-tree --merge-base=parent(new_sha) parent(old_sha) new_sha to
  produce the merged tree SHA. Same coerce-on-error → None
  semantics as Python (conflict, missing parents, old git
  without --write-tree all map to None so the caller falls back
  to the plain three-dot URL).
• compute_tree_delta — parses git diff-tree -r --raw --no-renames into TreeEntrys shaped for GitHub's
  POST /repos/.../git/trees. D status emits sha: None
  → JSON null to instruct GitHub to delete the path; other
  statuses we don't recognise are dropped on the floor (Python
  matches).
• mode_to_type — 160000→commit, 040000→tree,
  everything else→blob. Public so the eventual orchestrator
  can reuse it without a re-import.

Coverage: a real-repo orientation test that mirrors the bug
this whole module exists to fix (PR amended on a stable base —
the diff against parent_old^{tree} must contain ONLY the
amendment), plus conflict + missing-parent fallbacks and a
diff-tree parse over modified/added/deleted/type-changed
entries. Wire shape was cross-checked against the Python
implementation byte-for-byte.

The HTTP upload step (upload_replay_commit +
replay_for_revision) lands separately — it needs
mergify-core's HTTP client wired in, and the git half is
useful on its own when the eventual native stack push
arrives.

Co-Authored-By: Claude Opus 4.7 noreply@anthropic.com

Depends-On: #1533

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	feat(stack): port detect_change_type + patch-id helpers to Rust	#1532
2	feat(stack): port StackComment renderer to Rust	#1533
3	feat(stack): port replay git helpers (merge-tree + diff-tree) to Rust	#1534	👈
4	feat(stack): port RevisionHistoryComment to Rust	#1535
5	feat(stack): port stack push approvals decision to Rust	#1536
6	feat(stack): port replay HTTP upload + orchestrator to Rust	#1537
7	feat(stack): port stack push git plumbing (notes fetch + push) to Rust	#1539
8	feat(stack): port stack push rebase log lines to Rust	#1540
9	feat(stack): port format_pull_description + build_change_tasks to Rust	#1541
10	feat(stack): port create_or_update_pr + orphan branch delete to Rust	#1542
11	feat(stack): port per-PR stack + revision-history comment upserters to Rust	#1543
12	feat(stack): port stack push planner (get_changes) to Rust	#1544
13	feat(stack): port stack push orchestrator to Rust	#1545
14	feat(rust): wire native stack push + delete Python push.py	#1546
15	chore: delete Python tree + simplify wheel/CI for pure-Rust binary	#1547

[mergify]

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• depends-on=Mergifyio/mergify-cli#1533 [⛓️ feat(stack): port StackComment renderer to Rust #1533 [stack]]

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[jd]

Revision history

#	Type	Changes	Reason	Date
1	initial	97a4c1d		2026-06-04 14:36 UTC
2	content	97a4c1d → b652dd4 (raw)		2026-06-04 14:36 UTC