Johannes Sixt noticed that if the last file in the list was staged, my
earlier patch would display the diff for the penultimate file, but show
the file _before_ that as being selected.

This was due to my misunderstanding the lno argument to show_diff.

This patch fixes the problem: lno is not decremented in the special case
to handle the last item in the list (though we still need to use $lno-1
to find the right path for the next diff).
Signed-off-by: NAbhijit Menon-Sen <ams@toroid.org>
Tested-by: NJohannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>