Add buttons to allow loading of incomplete diffs (#16829)

This PR adds two buttons to the stats and the end of the diffs list to load the (some of) the remaining incomplete diff sections.

Contains #16775
    
Signed-off-by: Andrew Thornton <art27@cantab.net>


## Screenshots

### Show more button at the end of the diff
![Screenshot from 2021-09-04 11-12-37](https://user-images.githubusercontent.com/1824502/132091009-b1f6113e-2c04-4be5-8a04-b8ecea56887b.png)

### Show more button at the end of the diff stats box
![Screenshot from 2021-09-04 11-14-54](https://user-images.githubusercontent.com/1824502/132091063-86da5a6d-6628-4b82-bea9-3655cd9f40f6.png)

1 files changed, 24 insertions, 0 deletions

diff --git a/web_src/js/features/diff.js b/web_src/js/features/diff.js
new file mode 100644
index 0000000000..ef0aaceeeb
--- /dev/null
+++ b/web_src/js/features/diff.js
@@ -0,0 +1,24 @@
+export function initDiffShowMore() {
+  $('#diff-files, #diff-file-boxes').on('click', '#diff-show-more-files, #diff-show-more-files-stats', (e) => {
+    e.preventDefault();
+
+    if ($(e.target).hasClass('disabled')) {
+      return;
+    }
+    $('#diff-show-more-files, #diff-show-more-files-stats').addClass('disabled');
+
+    const url = $('#diff-show-more-files, #diff-show-more-files-stats').data('href');
+    $.ajax({
+      type: 'GET',
+      url,
+    }).done((resp) => {
+      if (!resp || resp.html === '' || resp.empty) {
+        $('#diff-show-more-files, #diff-show-more-files-stats').removeClass('disabled');
+        return;
+      }
+      $('#diff-too-many-files-stats').remove();
+      $('#diff-files').append($(resp).find('#diff-files li'));
+      $('#diff-incomplete').replaceWith($(resp).find('#diff-file-boxes').children());
+    });
+  });
+}