cherry_pick_list: quit early if one side is empty

cherry_pick_list: quit early if one side is empty

The --cherry-pick logic starts by counting the commits on each side,
so that it can filter away commits on the bigger one.  However, so
far it missed an opportunity for optimization: it doesn't need to do
any work if either side is empty.

This in particular helps the common use-case 'git rebase -i HEAD~$n':
it internally uses --cherry-pick, but since HEAD~$n is a direct
ancestor the left side is always empty.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>