git_sort: tests: Fix warning about default branch
    
    Since the version in SLE 15 git init prints this warning which is logged
    in the test result:
    
    hint: Using 'master' as the name for the initial branch. This default branch name
    hint: is subject to change. To configure the initial branch name to use in all
    hint: of your new repositories, which will suppress this warning, call:
    hint:
    hint:   git config --global init.defaultBranch <name>
    hint:
    hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
    hint: 'development'. The just-created branch can be renamed via this command:
    hint:
    hint:   git branch -m <name>
    
    The -b argument to git init to suppress this warning is not available on
    git versions that do not print the warning.
    
    pygit2.init_repository does not print this warning so use it instead.