Make --commit work even if repository has local changes

[PhilippWendler]

Currently benchexec --commit requires that the repository has no changes (untracked files are ok), otherwise it just does nothing. It should be enhanced to always commit the result files without changing anything else in the repository, and without committing additional files.

I think this is possible by adding the list of files not only to git add, but also to git commit, but we should check whether this guarantees the desired behavior.