tinydm-set-session: be quiet if already set

Don't print the "Session already set to" message if the new session is
the same as the old one. That way it doesn't get printed anymore
whenever upgrading one of the postmarketos-ui-* packages that use
tinydm.

tinydm-set-session.sh

@@ -26,6 +26,12 @@ mkdir -p /var/lib/tinydm
 
 target="/var/lib/tinydm/default-session.desktop"
 session_old=$(readlink -f $target)
+
+# Be quiet in post-install scripts if the desired session is already set
+if [ "$session_old" = "$session" ]; then
+	exit 0
+fi
+
 if [ -e "$session_old" ] && ! $force; then
 	echo "tinydm: Session already set to: $session_old"
         echo "tinydm: To change it, run: 'tinydm-set-session -f -s $session'"