Fix macOS and Windows CI: prevent parallel test temp-dir collisions in unique_temp_repo_dir

[Copilot]

The "Build (macOS)" CI job was failing because parallel unit tests could collide on the same temporary directory path, causing git init --bare to fail with File exists when copying templates into an already-initialized repo.

Root cause

unique_temp_repo_dir() used SystemTime::now().as_nanos() + pid — but all test threads share the same PID, and macOS system clock resolution can be coarser than 1 ns, so two concurrent calls could produce identical paths. add_origin_remote_with_feature() calls this twice per test; with branch_exists_anywhere_finds_remote_only_branch and branch_exists_anywhere_returns_false_for_truly_missing_branch running in parallel, one would win the git init --bare and the other would error.

Changes (tests/test_helpers.rs)

• Atomic counter replaces SystemTime + PID — an AtomicU64 incremented per call guarantees uniqueness regardless of timing or concurrency.

• Canonicalize temp dir base (non-Windows only) to resolve the macOS /var → /private/var symlink before constructing the path, preventing Homebrew git from encountering unexpected path aliasing during template copying. The canonicalize step is skipped on Windows because Path::canonicalize() there converts paths to the \\?\C:\... extended-length UNC format, which git rejects as an invalid hostname when used as a local remote URL and cannot resolve as a worktree path.

// Before
let ts = SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_nanos();
std::env::temp_dir().join(format!("mergetopus-test-{ts}-{}", std::process::id()))

// After
static UNIQUE_ID: AtomicU64 = AtomicU64::new(0);
let id = UNIQUE_ID.fetch_add(1, Ordering::Relaxed);
#[cfg(not(windows))]
let base = std::env::temp_dir().canonicalize().unwrap_or_else(|_| std::env::temp_dir());
#[cfg(windows)]
let base = std::env::temp_dir();
base.join(format!("mergetopus-test-{}-{}", std::process::id(), id))

[mwallner]

@copilot now the Windows tests are failing. All Actions must pass successfully.

[Copilot]

@copilot now the Windows tests are failing. All Actions must pass successfully.

Fixed in 58de74d. On Windows, std::path::Path::canonicalize() converts temp paths to the \\?\C:\... extended-length UNC format, which git rejects as an invalid hostname when used as a local remote URL, and cannot resolve as a worktree path. The fix wraps the canonicalize() call in #[cfg(not(windows))] so it only applies on macOS (where it's needed to resolve /var → /private/var) and is skipped on Windows.