github/workflows: fix codecov/codecov-action warning

Author: zchee

.github/workflows/test.yml

@@ -34,6 +34,8 @@ jobs:
 
     - name: Checkout code
       uses: actions/checkout@v2
+      with:
+        fetch-depth: 2
 
     - name: Install nvim binary
       uses: rhysd/action-setup-vim@v1