Merge branch 'ag/rebase-i-in-c' into js/rebase-in-c-5.5-work-with-rebase-i-in-c

* ag/rebase-i-in-c: rebase -i: move rebase--helper modes to rebase--interactive rebase -i: remove git-rebase--interactive.sh rebase--interactive2: rewrite the submodes of interactive rebase in C rebase -i: implement the main part of interactive rebase as a builtin rebase -i: rewrite init_basic_state() in C rebase -i: rewrite write_basic_state() in C rebase -i: rewrite the rest of init_revisions_and_shortrevisions() in C rebase -i: implement the logic to initialize $revisions in C rebase -i: remove unused modes and functions rebase -i: rewrite complete_action() in C t3404: todo list with commented-out commands only aborts sequencer: change the way skip_unnecessary_picks() returns its result sequencer: refactor append_todo_help() to write its message to a buffer rebase -i: rewrite checkout_onto() in C rebase -i: rewrite setup_reflog_action() in C sequencer: add a new function to silence a command, except if it fails rebase -i: rewrite the edit-todo functionality in C editor: add a function to launch the sequence editor rebase -i: rewrite append_todo_help() in C sequencer: make three functions and an enum from sequencer.c public