Force recursive clone in appveyor

appveyor.yml

@@ -1 +1,7 @@
-image: 'Visual Studio 2017'
+image: 'Visual Studio 2017'
+clone_script:
+- cmd: >-
+    git clone -q --branch=%APPVEYOR_REPO_BRANCH% https://github.com/%APPVEYOR_REPO_NAME%.git %APPVEYOR_BUILD_FOLDER%
+    cd %APPVEYOR_BUILD_FOLDER%
+    git checkout -qf %APPVEYOR_REPO_COMMIT%
+    git submodule update --init --recursive