mdt-29725: Pantheon upstream git detection.

Author: Naveen Valecha

assets/gitignore

@@ -88,3 +88,10 @@ public.key
 .lando/*.sh
 .lando/config/php/local_php.ini
 .local_dbs/
+
+#Pantheon upstream git detection.
+.github
+web/.gitignore
+web/sites/.gitignore
+web/sites/aliases.php
+web/sites/default/.gitignore