layout: improve dates logic

* BETTER Improves a test to display lastmod only if it's bigger than
publishdate.

Co-Authored-by: Igor Milhit <igor@milhit.ch>

layouts/partials/post-info.html

@@ -1,6 +1,6 @@
 <ul class="post-info">
   <li>Publié le&#x202F;: {{ dateFormat "2006-01-02" (default .Date (.PublishDate)) }}</li>
-  {{ if isset .Params "lastmod" }}
+  {{ if (and (isset .Params "lastmod") (gt .Lastmod .PublishDate)) }}
     <li>Dernière mise à jour&#x202F;: {{ .Lastmod.Format "2006-01-02" }}</li>
   {{ end }}
   {{ with .Params.categories }}