rebase-interactive: rewrite edit_todo_list() to handle the initial edit

edit_todo_list() is changed to work on a todo_list, and to handle the
initial edition of the todo list (ie. making a backup of the todo
list).

It does not check for dropped commits yet, as todo_list_check() does not
take the commits that have already been processed by the rebase (ie. the
todo list is edited in the middle of a rebase session).

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

sequencer.h

@@ -10,6 +10,7 @@ struct repository;
 const char *git_path_commit_editmsg(void);
 const char *git_path_seq_dir(void);
 const char *rebase_path_todo(void);
+const char *rebase_path_todo_backup(void);
 
 #define APPEND_SIGNOFF_DEDUP (1u << 0)