sequencer: directly call pick_commits() from complete_action()

Currently, complete_action(), used by builtin/rebase.c to start a new
rebase, calls sequencer_continue() to do it.  Before the former calls
pick_commits(), it

 - calls read_and_refresh_cache() -- this is unnecessary here as we've
   just called require_clean_work_tree() in complete_action()
 - calls read_populate_opts() -- this is unnecessary as we're starting a
   new rebase, so `opts' is fully populated
 - loads the todo list -- this is unnecessary as we've just populated
   the todo list in complete_action()
 - commits any staged changes -- this is unnecessary as we're starting a
   new rebase, so there are no staged changes
 - calls record_in_rewritten() -- this is unnecessary as we're starting
   a new rebase.

This changes complete_action() to directly call pick_commits() to avoid
these unnecessary steps.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.c

@@ -5140,15 +5140,21 @@ int complete_action(struct repository *r, struct replay_opts *opts, unsigned fla
 		return error_errno(_("could not write '%s'"), todo_file);
 	}
 
-	todo_list_release(&new_todo);
+	res = -1;
 
 	if (checkout_onto(r, opts, onto_name, &oid, orig_head))
-		return -1;
+		goto cleanup;
 
 	if (require_clean_work_tree(r, "rebase", "", 1, 1))
-		return -1;
+		goto cleanup;
 
-	return sequencer_continue(r, opts);
+	todo_list_write_total_nr(&new_todo);
+	res = pick_commits(r, &new_todo, opts);
+
+cleanup:
+	todo_list_release(&new_todo);
+
+	return res;
 }
 
 struct subject2item_entry {