Specify initial branch in tests

Without this, when a user changes the default branch name with
    git config --global init.defaultBranch main
the tests would fail.

Author: jdmansour

tests/repohelpers.py

@@ -18,7 +18,7 @@ def __init__(self, path=None):
 
     def __enter__(self):
         os.makedirs(self.path, exist_ok=True)
-        self.git('init', '--bare')
+        self.git('init', '--bare', '--initial-branch=master')
         return self
 
     def __exit__(self, *args):