Changing default branch makes all tests fail

### Bug description

If you have changed the default branch name on your computer, as `git init` now suggests, all nbgitpuller tests fail.

### How to reproduce

```bash
# The following fails:
git config --global init.defaultBranch main
python -m pytest tests/test_gitpuller.py

# This works again:
git config --global init.defaultBranch master
python -m pytest tests/test_gitpuller.py
```

I think this is because `repohelpers.py` hardcodes the master branch name, but when creating test repos doesn't specify a name. I'll look into it next week, it should not be too hard.