ci/source-check: check also the unstaged files

Prior this commit it was possible to pass the CI checks even without added
files in vendor directory, because git diff doesn't check for unstaged
files. This commit fixes it.

.github/workflows/tests.yml

@@ -28,10 +28,10 @@ jobs:
       - name: Check that source has been prepared
         run: |
           ./tools/prepare-source.sh
-          if ! git diff --quiet --exit-code ; then
+          if [ -n "$(git status --porcelain)" ]; then
             echo
             echo "Please include these changes in your branch: "
-            git diff
+            git status -vv
             exit "1"
           else
             exit "0"