feat(rust): port stack squash to native Rust

[jd]

The Rust binary now serves mergify stack squash <SRC>... into <TARGET> [-m <msg>] [--dry-run] natively. With this slice, the
entire stack rebase family is native: edit, drop,
fixup, reword, reorder, move, squash.

Final cleanup: mergify_cli/stack/squash.py,
mergify_cli/stack/reorder.py (the helpers squash.py was
still using), and their tests (test_squash.py,
test_squash_cli.py) are removed. mergify_cli/stack/cli.py
loses its squash click registration and its
_parse_squash_tokens helper.

The rebase-todo machinery grows one new variant —
Action::Squash — that combines what Reorder and
Fixup did separately plus an optional ExecAfter:

• ordered_shas — the full new pick order (target's neighbours
  rearranged so all sources sit directly after it).
• fixup_shas — the subset whose verb flips from pick to
  fixup.
• exec_after_sha + exec_command — when -m is given,
  an exec git commit --amend -F <file> is injected right
  after the last fixed-up source. The amend runs while HEAD
  still points at the combined target commit so
  prepare-commit-msg re-attaches the Change-Id.

mergify stack squash:

1. Resolves the trunk and walks the stack.
2. Parses SRC... into TARGET — the into keyword splits
   the positional list. Mirrors the Python parser:
   exactly one into, at least one source before, exactly
   one target after.
3. Validates: target is not among the sources; no source SHA
   appears twice.
4. Builds the new order: keep non-source commits in their
   original positions, insert all sources directly after
   target in the order they were listed.
5. --dry-run short-circuits with the plan; otherwise spawns
   git rebase -i <base> with the squash sequence editor.
6. With -m: writes the message to a leaked tempfile and
   passes it through --sha/--command on the
   rebase-todo-rewrite self-invocation.

End-to-end coverage in
crates/mergify-cli/tests/stack_squash.rs (6 cases): single
source into target keeps target's message, custom -m
replaces subject + body, dry-run is a no-op, source-equals-
target errors, missing into errors, multi-source folds all
into target. The pure transformer adds 3 new rebase_todo
unit tests (happy path, exec-after injection, count mismatch).

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

[jd]

This pull request is part of a Mergify stack:

#	Pull Request	Link
1	feat(rust): port stack squash to native Rust	#1521	👈
2	feat(rust): port stack checkout to native Rust	#1522
3	feat(rust): port stack sync to native Rust	#1523
4	feat(rust): port stack list + stack open to native Rust	#1524
5	feat(rust): port stack hooks + stack setup to native Rust	#1527
6	feat(rust): port stack push end-to-end + delete Python push.py	#1553
7	chore: delete Python tree + simplify wheel/CI for pure-Rust binary	#1554
8	refactor(stack): extract shared git helpers into crate::git	#1555
9	ci(workflows): align smoke-test-binary on python 3.14	#1557

[mergify]

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[jd]

Revision history

#	Type	Changes	Reason	Date
1	initial	9342e5c		2026-06-03 21:01 UTC
2	rebase	9342e5c → f86d325 (rebase only)		2026-06-03 21:01 UTC
3	rebase	f86d325 → beed698 (rebase only)		2026-06-04 08:51 UTC
4	rebase	beed698 → b5699bc (rebase only)		2026-06-04 13:29 UTC
5	rebase	b5699bc → 96311dd (rebase only)		2026-06-05 07:01 UTC
6	rebase	96311dd → 3116a40 (rebase only)		2026-06-05 08:13 UTC
7	rebase	3116a40 → ccfaf02 (rebase only)		2026-06-05 09:38 UTC
8	rebase	ccfaf02 → bdf0e4a (rebase only)		2026-06-05 12:37 UTC
9	rebase	bdf0e4a → b0223f3 (rebase only)		2026-06-08 11:31 UTC

The base branch was changed.

[mergify]

@jd this pull request is now in conflict 😩

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-06-08 14:48 UTC · Rule: default
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-06-08 14:49 UTC · at b0223f39130ed27b1db2ab26d882194f9e448c45 · squash

This pull request spent 1 minute 19 seconds in the queue, including 8 seconds running CI.

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