Use git status instead of ls-files (error on flat maven structure)

Hi,

I would like to use maven-gitflow-plugin in project but it has a bit unusual structure (flat). You can find example here:
https://github.com/antusus/release-test

In short root pom is in base module. I'm invoking maven commands from base folder:
```
cd base
mvn gitflow:release-start
```

In version 1.0.1-alpha2 when I executed release-start there was an error that I have untracked files. But git status was telling me that everything is clean. The problem was caused by "git ls-files --error-unmatch" flag. It happened because git command was not executed in root folder but in one of sub modules. When I changed that to "git status --short" everything is working again. Maybe you could change ls-files to status?