Fix behavior of --todo with an optional argument

The previous behavior of --todo was to always only list uncompleted
items, unless zero was specified as additional argument. Restore and
document this behavior.

Also, fix two tests that failed because the --todo output is now sorted.

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

test/filter-001.sh

@@ -8,8 +8,8 @@ if [ "$1" = 'actual' ]; then
 elif [ "$1" = 'expected' ]; then
   cat <<EOD
 to do:
-9. Gloriously slams
 9. Beefburger's
+9. Gloriously slams
 9. Seasons
 
 02/23/13: