Don't show buffer if it's not listed

This prevents deleted or hidden buffers to show up
in buffers segment for single tab or current tab.

When working with lot of buffers and closing them on regular basis,
or when buffers where created by plugins (NERDTree for example)
I ended up with lots of entries in buffer line, although :buffers command
didn't show them.
You must use tabs for indentation. (Though I would not edit file at all: `list_all_buffers` looks like the option that is not needed most of time.)