When a desktop is preseeded, only enable that desktop.

debian/changelog

@@ -1,11 +1,15 @@
 tasksel (3.30) UNRELEASED; urgency=medium
 
+  [ Joey Hess ]
   * Remove pm-utils from laptop and desktop tasks. It was only added because
     hal needed it, hal doesn't exist anymore, and systemctl is used for
     command-line suspend/hibernate now.
   * Remove libgl1-mesa-dri from the desktop task list. xorg depends on it,
     so it does not need to be explicitly listed.
 
+  [ Samuel Thibault ]
+  * When a desktop is preseeded, only enable that desktop. Closes: #771576.
+
  -- Joey Hess <joeyh@debian.org>  Thu, 30 Oct 2014 15:58:02 -0400
 
 tasksel (3.29) unstable; urgency=high

tests/default-desktop

@@ -27,8 +27,10 @@ check_desktop_wanted() {
 	fi
 }
 
-if db_get "tasksel/desktop" && echo "$RET" | grep -q "$DESKTOPNAME"; then
-	check_desktop_wanted
+if db_get "tasksel/desktop"; then
+	if echo "$RET" | grep -q "$DESKTOPNAME"; then
+		check_desktop_wanted
+	fi
 else
 	if [ "$DESKTOPNAME" = "$(default_desktop_for_arch $(dpkg --print-architecture))" ]; then
 		check_desktop_wanted