hgweb: add ajaxScrollInit function, which does the ajax requests and processing

This function should be correctly called on a page, otherwise there is
no effect.
When called, it makes ajax requests for the next portion of changesets when the
user scrolls to the end. Also, when the monitor is high so that the
default amount of changesets isn't enough to fill it, multiple portions are
loaded if needed after the page load.

mercurial/templates/static/mercurial.js

@@ -345,3 +345,61 function docFromHTML(html) {
 function appendFormatHTML(element, formatStr, replacements) {
     element.insertAdjacentHTML('beforeend', format(formatStr, replacements));
 }
+
+function ajaxScrollInit(urlFormat,
+                        nextHash,
+                        nextHashRegex,
+                        containerSelector,
+                        messageFormat) {
+    updateInitiated = false;
+    container = document.querySelector(containerSelector);
+
+    function scrollHandler() {
+        if (updateInitiated) {
+            return;
+        }
+
+        var scrollHeight = document.documentElement.scrollHeight;
+        var clientHeight = document.documentElement.clientHeight;
+        var scrollTop = document.body.scrollTop
+            || document.documentElement.scrollTop;
+
+        if (scrollHeight - (scrollTop + clientHeight) < 50) {
+            updateInitiated = true;
+
+            if (!nextHash) {
+                return;
+            }
+
+            makeRequest(
+                format(urlFormat, {hash: nextHash}),
+                'GET',
+                function onstart() {
+                },
+                function onsuccess(htmlText) {
+                    var m = htmlText.match(nextHashRegex);
+                    nextHash = m ? m[1] : null;
+
+                    var doc = docFromHTML(htmlText);
+                    var nodes = doc.querySelector(containerSelector).children;
+                    while (nodes.length) {
+                        var node = nodes[0];
+                        node = document.adoptNode(node);
+                        container.appendChild(node);
+                    }
+                    process_dates();
+                },
+                function onerror(errorText) {
+                },
+                function oncomplete() {
+                    updateInitiated = false;
+                    scrollHandler();
+                }
+            );
+        }
+    }
+
+    window.addEventListener('scroll', scrollHandler);
+    window.addEventListener('resize', scrollHandler);
+    scrollHandler();
+}