Attempt to make submodules work with cirrus.

.cirrus.yml

@@ -2,7 +2,18 @@ freebsd_instance:
   image: freebsd-12-0-release-amd64
 
 task:
-  install_script: pkg install -y cmake ninja llvm80
+  install_script: pkg install -y cmake ninja llvm80 git
+
+  clone_script: |
+    if [ -z "$CIRRUS_PR" ]; then
+      git clone --recursive --branch=$CIRRUS_BRANCH https://x-access-token:${CIRRUS_REPO_CLONE_TOKEN}@github.com/${CIRRUS_REPO_FULL_NAME}.git $CIRRUS_WORKING_DIR
+      git reset --hard $CIRRUS_CHANGE_IN_REPO
+    else
+      git clone --recursive https://x-access-token:${CIRRUS_REPO_CLONE_TOKEN}@github.com/${CIRRUS_REPO_FULL_NAME}.git $CIRRUS_WORKING_DIR
+      git fetch origin pull/$CIRRUS_PR/head:pull/$CIRRUS_PR
+      git reset --hard $CIRRUS_CHANGE_IN_REPO
+    fi
+
   script: |
       mkdir Build
       cd Build