Optimize rebase/squash rewrite performance and expand regression coverage

[svarlamov]

Summary

This PR substantially reduces git-ai overhead for rewrite workflows (rebase, merge --squash, and related post-commit paths), while tightening correctness with targeted regression tests.

The work is focused on first-principles performance improvements:

• reduce total work done,
• reduce number of git subprocesses,
• avoid content hydration unless strictly necessary,
• preserve existing authorship semantics.

Problem

In large repos and rewrite-heavy flows (Graphite stacks, large generated diffs, long commit ranges), rewrite-related operations were dominated by:

• many small sequential git calls,
• broad-path scans instead of staged/changed subsets,
• expensive attribution/materialization on paths that cannot affect AI output,
• avoidable per-commit/per-file note/path parsing.

This led to multi-minute stalls in real rebase/squash scenarios.

Design/Approach

1) Shrink worksets to AI-relevant + staged/changed paths

• Squash prep now starts from actual staged files after merge --squash instead of broad branch diffs.
• Path sets are narrowed to files touched by AI notes in relevant commit ranges.
• Small staged-set optimization bypasses expensive source-side file extraction when safe.

2) Batch git object reads

• Added batched content readers using git cat-file --batch:
  • Repository::get_files_content_at_commit
  • optimized Repository::get_all_staged_files_content
• Rebase/squash and traversal code now hydrate blobs in bulk rather than per-file calls.

3) Fast-paths for no-op / metadata-only scenarios

• Rebase note remap fast path for unchanged tracked blobs.
• Squash prep direct-reuse path when target AI side is empty and staged content matches source AI state.
• Metadata-only note remap retained where no AI-touched files need full rewrite.

4) Avoid unnecessary heavy attribution/materialization

• VirtualAttributions now supports line-only/initial-only loaders for post-commit paths:
  • from_just_working_log_line_only
  • from_initial_only_line_only
• This avoids eager file-content + char-range hydration where line-level data is sufficient.
• Blame path adds machine-mode fast behavior (skip presentation-only human-author hydration when not needed).

5) Squash pre-commit checkpoint skip handshake

• Squash prep stores staged index tree OID marker.
• Pre-commit checkpoint fast path skips when tree is unchanged and INITIAL already contains needed AI attribution.
• Post-commit consumes INITIAL directly in this path.
• Marker lifecycle is cleaned/reset correctly.

Correctness & Semantics

Key invariant preserved:

• We do not change authorship semantics to chase speed.

Notable fix included:

• Restored virtual-attribution file tracking semantics so files are not dropped just because blamed AI lines are empty (this previously caused note-loss regression in certain rebase cases).

Test Coverage Added

New edge/regression tests cover:

• squash pre-commit skip marker match/mismatch behavior,
• squash marker persistence + cleanup lifecycle,
• post-commit filtering predicate correctness for human/AI/override paths,
• line-attribution compression merge boundaries,
• VirtualAttributions initial-only and message-clear behavior,
• authorship traversal parser robustness (quoted paths, truncated batch payload),
• batched repository readers and index-tree OID behavior.

Representative test additions:

• src/commands/checkpoint.rs
• src/git/repo_storage.rs
• src/authorship/post_commit.rs
• src/authorship/rebase_authorship.rs
• src/authorship/virtual_attribution.rs
• src/git/authorship_traversal.rs
• src/git/repository.rs

Bench/Diagnostics

• Added benchmark harness for heavy squash scenarios:
  • scripts/benchmarks/git/benchmark_nasty_squashes.sh
• Added/expanded perf logging around rewrite/post-commit hot paths to make bottlenecks explicit.

Validation

Ran full suite successfully:

• cargo test -- --test-threads=1

No test failures.

Scope

Primary touched areas:

• rewrite orchestration (rebase_authorship, squash prep/rewrite)
• virtual attribution loading/conversion paths
• blame/traversal batching optimizations
• checkpoint/post-commit fast paths
• repository/storage helpers for batch reads and squash markers

---

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