curds: Forbid selecting the Previous playlist

I guess this could be lifted later if anybody asks for it, but I don't
think this really makes sense as a playlist source.

Signed-off-by: Anna Schumaker <Anna@NoWheyCreamery.com>

curds/playlist/manager.py

@@ -155,6 +155,8 @@ class PlaylistManager(PlaylistManagerBase):
 
         if plist == self.lookup("Collection"):
             self.current = [ ]
+        elif plist == self.lookup("Previous"):
+            return
         elif plist in self.current:
             self.current.remove(plist)
         self.current.insert(0, plist)

curds/playlist/test_manager.py

@@ -251,6 +251,11 @@ class TestPlaylistManager(unittest.TestCase):
         self.assertEqual(self.playman.current, [ plist, glist, clist ])
         self.assertEqual(self.cb_change[-2:], [ glist, plist ])
 
+        self.cb_change = [ ]
+        self.playman.select(self.playman.lookup("Previous"))
+        self.assertEqual(self.playman.current, [ plist, glist, clist ])
+        self.assertEqual(self.cb_change, [ ])
+
         plist.current = 1247
         peek = self.playman.peek(4)
         self.assertEqual(len(peek), 4)