rebase-interactive: append_todo_help() changes

rebase-interactive: append_todo_help() changes

This moves the writing of the comment "Rebase $shortrevisions onto
$shortonto ($command_count commands)" from todo_list_write_to_file() to
append_todo_help().

shortrevisions, shortonto, and command_count are passed as parameters to
append_todo_help().

During the initial edit of the todo list, shortrevisions and shortonto
are not NULL.  Therefore, if shortrevisions or shortonto is NULL, then
edit_todo would be true, otherwise it would be false.  Thus, edit_todo
is removed from the parameters of append_todo_help().

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