Correctly display track number 0 in show_change

fixes issue beetbox#3346: For (hidden) tracks with index 0, the UI would
previously show a #0 -> beetbox#1 change when actually the index would be set to 0.
Now, properly distinguish index 0 and None (i.e. not set)

beets/ui/commands.py

@@ -241,7 +241,7 @@ def format_index(track_info):
             if mediums and mediums > 1:
                 return u'{0}-{1}'.format(medium, medium_index)
             else:
-                return six.text_type(medium_index or index)
+                return six.text_type(medium_index if medium_index is not None else index)
         else:
             return six.text_type(index)