Merge branch 'ag/sequencer-reduce-rewriting-todo'

Merge branch 'ag/sequencer-reduce-rewriting-todo'

The scripted version of "git rebase -i" wrote and rewrote the todo
list many times during a single step of its operation, and the
recent C-rewrite made a faithful conversion of the logic to C.  The
implementation has been updated to carry necessary information
around in-core to avoid rewriting the same file over and over
unnecessarily.

* ag/sequencer-reduce-rewriting-todo:
  rebase--interactive: move transform_todo_file()
  sequencer: use edit_todo_list() in complete_action()
  rebase-interactive: rewrite edit_todo_list() to handle the initial edit
  rebase-interactive: append_todo_help() changes
  rebase-interactive: use todo_list_write_to_file() in edit_todo_list()
  sequencer: refactor skip_unnecessary_picks() to work on a todo_list
  rebase--interactive: move rearrange_squash_in_todo_file()
  rebase--interactive: move sequencer_add_exec_commands()
  sequencer: change complete_action() to use the refactored functions
  sequencer: make sequencer_make_script() write its script to a strbuf
  sequencer: refactor rearrange_squash() to work on a todo_list
  sequencer: refactor sequencer_add_exec_commands() to work on a todo_list
  sequencer: refactor check_todo_list() to work on a todo_list
  sequencer: introduce todo_list_write_to_file()
  sequencer: refactor transform_todos() to work on a todo_list
  sequencer: remove the 'arg' field from todo_item
  sequencer: make the todo_list structure public
  sequencer: changes in parse_insn_buffer()