Remove ignored files from ffetch stash

Hi @gastonmancini, @matiasbeckerle and @jfazzini

@Gkolocsar has detected an issue with stashing ignored files:

```console
$ git ffetch
Fetching origin
Fetching upstream
...
error: open("D_Data/D_Data.dbmdl"): Permission denied
fatal: Unable to process path D_Data/D_Data.dbmdl
Cannot save the untracked files
M       Application/Alerts/AlertService.cs
Switched to a new branch '__FFETCH_AUX'
...
Deleted branch __FFETCH_AUX (was 6befcda).
No stash found.
```

So, maybe the best option is to _ignore ignored files_. Do you agree?