Git status decorations

Show Git state next to files:

```bash
treels --git-status
```

Example:

```text
.
├── M README.md
├── ? new-file.go
└──   main.go
```

Possible symbols:

| Symbol | Meaning |
| --- | --- |
| `M` | modified |
| `A` | added |
| `D` | deleted |
| `?` | untracked |
| `!` | ignored