perf: skip managed repo lookup for intermediate post-checkout during rebase

[svarlamov]

perf: skip managed repo lookup for intermediate post-checkout during rebase

Summary

Fixes the rebase_merges / current_hooks margin check failure from PR #530 (27.255% > 25.0% threshold).

During --rebase-merges, git fires post-checkout for every intermediate checkout operation (topology recreation). In hooks mode, each invocation spawns a new process that:

1. Reads the state JSON (should_forward_repo_state_first)
2. Opens the git repository (find_hook_repository_from_context)
3. Runs managed hook logic that is effectively a no-op during rebase

Two optimizations:

1. Skip repo lookup for intermediate post-checkout during rebase — hook_requires_managed_repo_lookup now returns false for post-checkout when a rebase is in progress, unless it's a terminal event (rebase --abort or pull --rebase fallback). This matches wrapper-mode behavior where intermediate checkout events don't get individual checkout hook processing.

2. Avoid redundant state file re-read — When should_forward_repo_state_first(None) already returned None (no forward target) and no repo was opened, skip the call to execute_forwarded_hook which would re-read the same state file.

Local nasty benchmark result after fix: 6/6 margin checks passing (hooks rebase_merges dropped from ~27% to ~11% slowdown vs wrapper).

Review & Testing Checklist for Human

• Verify skipping checkout_hooks::post_checkout_hook() for intermediate rebase checkouts is safe. The run_managed_hook("post-checkout") handler does real work including calling checkout_hooks::post_checkout_hook(). By skipping the repo lookup, this entire handler is skipped for non-terminal rebase checkouts. Confirm this is consistent with wrapper-mode behavior (where the wrapper handles rebase as one command, not per-checkout).
• Verify the abort/pull guards are complete. The skip only fires when !is_rebase_abort_reflog_action() && !is_pull_reflog_action(). Are there any other terminal rebase events that arrive via post-checkout and need managed processing?
• Verify the early-return optimization (cached_forward_dir.is_none() && repo.is_none()) doesn't break edge cases where GIT_DIR might not be set in the hook environment (e.g., worktrees, unusual git configurations). The assumption is that the env-based state file lookup is authoritative when repo is None.
• Run the nasty benchmark a few times to confirm the fix consistently passes the 25% margin (CI only runs 1 repetition so variance is high).

Notes

• The 6 test failures in the local cargo test run are pre-existing on main (snapshot version mismatches and a network error) — not introduced by this change.
• Link to Devin run: https://app.devin.ai/sessions/2ae70595150c4b718441b1a3c599eeda
• Requested by: @svarlamov

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[svarlamov]

devin fix the lints