Failed to catch the filename when grepping a file(NOT folder) with gnu-grep

Grepper works well with folder:

```bash
:Grepper -tool grep -noprompt -quickfix -highlight -grepprg grep -n -i name ./src
```

While works bad with file:
```bash
:Grepper -tool grep -noprompt -quickfix -highlight -grepprg grep -n -i name ./package.json
```
which will produce a quickfix like(noticed that filename is not captured):
```text
 2:  "name": "bank-demo",||
```
