This function simply returns the current tabnumber followed by the
number of buffers in the current tabpage.
The previous one either was broken long ago or did not work as expected.
fixes#2616
Make sure, that the second argument is actually a number and not a list.
Note: even when fixing this, the tabpage still shows some strange output
e.g. currently
1 1.%{len(tabpagebuflist(1))}
In a vim window. Not sure, why Vim does no longer evaluate those %{..}
sections. Perhaps some change in Vim?
closes#2616
