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

revision.c

@@ -514,6 +514,9 @@ static void cherry_pick_list(struct commit_list *list, struct rev_info *revs)
 			right_count++;
 	}
 
+	if (!left_count || !right_count)
+		return;
+
 	left_first = left_count < right_count;
 	init_patch_ids(&ids);
 	if (revs->diffopt.nr_paths) {