sequencer: move the code writing total_nr on the disk to a new function

sequencer: move the code writing total_nr on the disk to a new function

The total number of commands can be used to show the progression of the
rebasing in a shell.  It is written to the disk by read_populate_todo()
when the todo list is loaded from sequencer_continue() or
pick_commits(), but not by complete_action().

This moves the part writing total_nr to a new function so it can be
called from complete_action().

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