--extend-exclude doesn't seem to work

```bash
mkdir -p repro/vendor && cd repro
printf 'let  x   =   1\n' > vendor/dep.nim
nph --check --extend-exclude:'vendor/' .
```

Output:

```bash
would reformat ./vendor/dep.nim

Oh no! 💥 💔 💥
1 file would be reformatted.
```