sequencer: changes in parse_insn_buffer()

sequencer: changes in parse_insn_buffer()

This clears the number of items of a todo_list before parsing it to
allow to parse the same list multiple times without issues.  As its
items are not dynamically allocated, or don’t need to allocate memory,
no additionnal memory management is required here.

Furthermore, if a line is invalid, the type of the corresponding
command is set to a garbage value, and its argument is defined properly.
This will allow to recreate the text of a todo list from its commands,
even if one of them is incorrect.

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