[CI] Setup default git branch name (#82)

To squash annoying warnings during the testing

.github/workflows/CI.yml

@@ -40,6 +40,7 @@ jobs:
         run: |
           git config --global user.email "[email protected]"
           git config --global user.name "mr fake"
+          git config --global init.defaultBranch main
       - name: Install and initialize sapling on macos
         if: matrix.os == 'macos-latest'
         run: |