F-020: perf(services): semaphore-bound concurrent git show spawning

[Sephyi]

Summary

perf(services): semaphore-bound concurrent git show spawning.

Audit context

Closes audit entry F-020 from #3.

Verification

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-targets

Note: one pre-existing test porcelain_exits_within_timeout_with_no_staged_changes is a known macOS cold-start flake that reproduces on unmodified development — unrelated to this change.

[Copilot]

Pull request overview

This PR addresses audit finding F-020 by adding a concurrency bound around git show subprocess spawning in GitService::fetch_file_contents, preventing large staged sets from triggering unbounded process creation.

Changes:

• Introduces a computed concurrency limit for git show subprocesses.
• Wraps per-file content fetching in a tokio::sync::Semaphore permit to bound concurrent git show execution.

Comments suppressed due to low confidence (1)

src/services/git.rs:265

• The semaphore is acquired inside each spawned task, so fetch_file_contents still creates one task per path immediately. On very large stages this can allocate a large number of pending tasks even though subprocess concurrency is bounded. If the goal is to bound overall work, consider acquiring the permit before spawning and moving it into the task (so only up to N tasks exist at once), or switching to a bounded-concurrency pattern that doesn't enqueue all tasks at once.

        let mut set = tokio::task::JoinSet::new();
        let work_dir: Arc<PathBuf> = Arc::new(self.work_dir.clone());
        let semaphore = Arc::new(tokio::sync::Semaphore::new(
            Self::git_show_concurrency_limit(),
        ));

        for path in paths {
            let work_dir = Arc::clone(&work_dir);
            let semaphore = Arc::clone(&semaphore);
            let path = path.clone();
            set.spawn(async move {
                // Semaphore is never closed, so acquire cannot fail.
                let _permit = semaphore
                    .acquire_owned()
                    .await
                    .expect("git-show semaphore closed unexpectedly");
                let staged =
                    Self::fetch_git_show(&work_dir, &format!(":0:{}", path.display())).await;
                let head =
                    Self::fetch_git_show(&work_dir, &format!("HEAD:{}", path.display())).await;
                (path, staged, head)
            });

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