Changing default branch makes all tests fail

[jdmansour]

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

# 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.

[consideRatio]

Ah nice work figuring that out!!