Rewrite of the remaining "rebase -i" machinery 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
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