Add `gitignore` option for automatic gitignore-aware file hiding

Feature description

Add a new boolean option gitignore (default false). When enabled, lf automatically hides files ignored by .gitignore when inside a git worktree, and falls back to normal hiddenfiles behavior otherwise.

Why this cannot be done in user configuration

This was the first thing I tried :

cmd on-cd %{{
    if git rev-parse --is-inside-work-tree >/dev/null 2>&1; then
        IGNORED=$(ls -A | git check-ignore --stdin 2>/dev/null | tr '\n' ':')
        lf -remote "send $id set hiddenfiles '.git:${IGNORED}'"
    else
        lf -remote "send $id set hiddenfiles '.*'"
    fi
}}

The problem: on-cd runs in a subprocess that does not block the UI thread. By the time lf -remote delivers the set hiddenfiles command, lf has already rendered the directory using the old patterns. The subsequent remote command forces a re-sort and redraw, producing a visible blink/flash every time you change directories. Caching the value (comparing old vs new) only reduces redundant calls -- it cannot eliminate the IPC delay inherent to the hook architecture.

Proposed implementation

The fix belongs at the directory-loading layer (nav.go), before the UI ever sees the files:

1. getGitIgnored(dir, names) -- calls git check-ignore --stdin -z once per directory load. Returns a map[string]bool of ignored basenames. If the directory is not inside a git worktree (or git is not installed), returns nil.
2. readdir -- populates file.gitIgnored for each file when inside a worktree.
3. dir.sort() -- uses a mutually exclusive filter:
   • Inside a git worktree: hide only gitIgnored files (and .git).
   • Outside a worktree: use normal hiddenfiles patterns.
4. checkDir -- detects gitignore option toggles and triggers async reloads via the existing directory reload infrastructure (same pattern used for dircounts changes).

Changes

Complexity assessment

• No new Go dependencies. nav.go already imports os/exec.
• No new goroutines. The git check runs synchronously inside the existing directory-loading goroutine.
• Performance: One git check-ignore --stdin -z call per directory load. Negligible overhead when disabled. When enabled, batching via stdin is faster than N individual git check-ignore calls.
• Platform: Pure Go, works on any platform with git in $PATH.

Usage

set hiddenfiles ".*"
set gitignore

• Inside a git repo: only gitignored files (plus .git) are hidden. Normal dotfiles (e.g. .bashrc) that are not ignored remain visible.
• Outside a git repo: normal hiddenfiles behavior applies.
• Toggle at runtime with :set gitignore!.