git: Add more features to tag.sh script

This helps me make releases and probably won't help you, but why not be
transparent about things and tools!

tag.sh

@@ -9,3 +9,5 @@ echo "Press ^C within 3s to abort."
 sleep 3s
 echo "release: tag $t" | git tag --file=- --sign $t
 git push origin $t
+git diff --stat "$v" "$t"
+if which contrib.sh 2>/dev/null; then contrib.sh "$v"; fi