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>