simplify trail worktree checkout internals

Reuse existing package helpers instead of local duplicates: shellQuote, parseWorktreeForBranch, remote.Fetch (regaining GIT_TERMINAL_PROMPT=0, --no-auto-gc, and the consistent fetch-timeout message), and the runGit test helper. Resolve the main worktree root once per invocation instead of re-spawning git rev-parse up to three times, and drop the constant rule parameter from appendIgnoreRule.