tests: clarify that default parameters are for form - direct model access requires different types _get_repo_group_create_params parent_group_id is thus set to the form value '-1' instead of None. It worked before anyway because the model failed to find the repo '-1' and thus got pretty much the same as None.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.