Add a tracking pixel with JavaScript (jenkinsci#1800)

* Add a tracking pixel with JavaScript

* Remove prior attempt at tracking pixel

Author: MarkEWaite

README.adoc

@@ -3,11 +3,14 @@
 :toc: macro
 :toc-title:
 
++++
+<script src="images/scarf.js"></script>
++++
+
 [#introduction]
 == Introduction
 
 image::images/jenkins-and-git.png[Jenkins and Git]
-image::https://static.scarf.sh/a.png?x-pxid=e010adc8-3614-41ef-b7e8-2f42328e8962&page=git-plugin-readme[]
 
 The git plugin provides fundamental git operations for Jenkins projects.
 It can poll, fetch, checkout, branch, list, merge, tag, and push repositories.

images/scarf.js

@@ -0,0 +1,29 @@
+(function () {
+    const pixelID = 'e011adc8-3614-41ef-b7e8-2f42328e8962';
+    let lastHref = null;
+
+    function sendScarfPing() {
+        const currentHref = window.location.href;
+        if (currentHref === lastHref) return;
+        lastHref = currentHref;
+
+        const url = `https://static.scarf.sh/a.png?x-pxid=${pixelID}`;
+        const img = new Image();
+        img.referrerPolicy = 'no-referrer-when-downgrade';
+        img.src = url;
+    }
+
+    ['pushState', 'replaceState'].forEach(fn => {
+        const original = history[fn];
+        history[fn] = function () {
+            original.apply(this, arguments);
+            window.dispatchEvent(new Event('scarf:locationchange'));
+        };
+    });
+
+    window.addEventListener('hashchange', sendScarfPing);
+    window.addEventListener('popstate', sendScarfPing);
+    window.addEventListener('scarf:locationchange', sendScarfPing);
+
+    sendScarfPing(); // initial page load
+})();