Use git last modified as date

src/blog/posts/posts.11tydata.js

@@ -1,5 +1,6 @@
 module.exports = {
     tags: ['posts'],
     layout: 'post.njk',
-    permalink: "/post/{{ data.slug if data.slug else title | slugify  }}/"
+    permalink: "/post/{{ data.slug if data.slug else title | slugify  }}/",
+    date: "git Last Modified"
 }