sequencer: refactor append_todo_help() to write its message to a buffer

sequencer: refactor append_todo_help() to write its message to a buffer

This refactors append_todo_help() to write its message to a buffer
instead of the todo-list.  This is needed for the rewrite of
complete_action(), which will come after the next commit.

As rebase--helper still needs the file manipulation part of
append_todo_help(), it is extracted to a temporary function,
append_todo_help_to_file().  This function will go away after the
rewrite of complete_action().

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