Meta: Ensure local WPT repository is up to date before importing

The current import process assumes that the local WPT repository is up
to date. Let's make sure that's true before importing a new test.

Meta/WPT.sh

@@ -578,6 +578,22 @@ list_tests_wpt()
 
 import_wpt()
 {
+    pushd "${WPT_SOURCE_DIR}" > /dev/null
+       if ! git fetch origin > /dev/null; then
+            echo "Failed to fetch the WPT repository, please check your network connection."
+            exit 1
+        fi
+        local local_hash
+        local_hash=$(git rev-parse HEAD)
+        local remote_hash
+        remote_hash=$(git rev-parse origin/master)
+
+        if [ "$local_hash" != "$remote_hash" ]; then
+            echo "WPT repository is not up to date, please run '$0 update' first."
+            exit 1
+        fi
+    popd > /dev/null
+
     for i in "${!INPUT_PATHS[@]}"; do
         item="${INPUT_PATHS[i]}"
         item="${item#http://wpt.live/}"