perf: include the latest posts in the "Recently Updated" list (#1456)

Improve the "Recently Updated" list to include the most recent (only one Git commit) posts.

This change has two benefits:

1. The post update list is consistent with the git commit timeline.
2. Avoid users not having a list of updates to show when they first publish a site.

_includes/update-list.html

@@ -1,16 +1,17 @@
-<!-- Get the last 5 posts from lastmod list. -->
+<!-- Get 5 last posted/updated posts -->
 
 {% assign MAX_SIZE = 5 %}
 
 {% assign all_list = '' | split: '' %}
 
 {% for post in site.posts %}
-  {% if post.last_modified_at and post.last_modified_at != post.date %}
+  {% assign datetime = post.last_modified_at | default: post.date %}
+
   {% capture elem %}
-      {{- post.last_modified_at | date: "%Y%m%d%H%M%S" -}}::{{- forloop.index0 -}}
+    {{- datetime | date: "%Y%m%d%H%M%S" -}}::{{- forloop.index0 -}}
   {% endcapture %}
+
   {% assign all_list = all_list | push: elem %}
-  {% endif %}
 {% endfor %}
 
 {% assign all_list = all_list | sort | reverse %}