Reduce unnecessary reading of state variables back from the disk
during sequencer operation.
* ag/sequencer-todo-updates:
sequencer: directly call pick_commits() from complete_action()
rebase: fill `squash_onto' in get_replay_opts()
sequencer: move the code writing total_nr on the disk to a new function
sequencer: update `done_nr' when skipping commands in a todo list
sequencer: update `total_nr' when adding an item to a todo list